Examples:  a noticeable decline in quality

To see the results of 1968, simply pick up three books. you can get them pretty cheap, especially second hand. You need Essential Fantastic Four volumes 4,5 and 6: before, during and after 1968.

Essential FF Volume 4: 1966-68.
This volume covers the last of the pre-1968 issues. Note the number of major new characters and major character development.
64: The Kree! the Sentry!
65: Ronan! The Supreme Intelligence!
66-7: Him! (Later to become Adam Warlock.) The beehive!
Annual 5: The Psychoman! The Microverse! Quasimodo! Sue is pregnant!
68-71: The Thing versus the FF: after so much change Kirby can be forgiven a straight fight.
72: The Silver Surfer: adds depth.
73: big crossover battle issue. Unlike today, this was a new idea!
74-77: Galactus returns, but with new twists (and only his first return, so the idea is still fresh)
78-79: Wonder gloves, and depth added to Ben Grimm's character.
80: The Living Totem!
Annual 6: This is the big one! The Negative Zone,Annihilus, and the birth of Franklin!
81: Crystal joins the team! A major development (and a great ending)!
82-83: Maximus. Crystal and Johnny are reunited, but otherwise it's a low detail, low creativity story. 1968 has arrived.

Essential FF Volume 5: 1968-70.
This volume covers the last Lee-Kirby issues. This is when Kirby began withholding his best stuff (such as the New Gods he was planning at the time), and was planning to leave Marvel.

84-87: Doom. Old character, but a classic, unforgettable story.
88-89: Mole Man's house. Does a house count as a character? Full of new ideas.
90-93: Skrull world. Mostly old characters but some new, and lots of creativity.
94: Agatha Harkness: a classic! And all done in one issue!
95: Monacle: not a favorite characters, but still creative
96: Mad Thinker and fake FF. First non-creative Kirby story? (Evil FF used in last Galactus story)
97: Monster from lost lagoon: Creative and done in one. Very nice.
98: American moon landing story. Creative (or at least historic) and done in one.
99: Torch rescues Crystal. Some creativity (e.g. the children) and story development. Done in one.
100: Fun anniversary issue.  Done in one.
101: Bedlam at Baxter Building. Some creativity (e.g. memorable Maggia). Done in one.
102-4: Namor and Magneto. 103 is first non-Kirby issue. No depth or creativity.
105-6: New "monster" - mildly creative, and first Sue in flared trousers!
107-10: Janus. And interesting case: Jack is briefly back, and Stan butchers his 108 story. The original, finally printed in 2009, is a creative story of introducing Janus and an ancient artifact. Stan makes it a standard Negative Zone related story.
Conclusion:
When Kirby is there we see a little less creativity and depth as the early days, but each story takes  a little longer to tell.

Essential FF  Volume 6: 1970-72
This includes the first of the next generation writers and artists: the Thomas-Buscema issues.
111: Thing runs amok - it's happened at least twice before (circa FF 40, and 68-69)
112: Hulk V Thing - happened twice before (issue 10, and Hulk's mag)
113-116: Overmind: old story (Galactus), new names, and less believable. Unrealistic use of Doom.
117-118: Diablo and Crystal in Central America - old characters new locations.
119: Panther in South Africa stand-in - old characters new locations.
120-123: extremely poor retelling of Galactus. Noby comes out of this looking good.
124-125: retells Lost Lagoon.
126: retells issue 1.
127-128: Mole Man and Kala. Old characters, but adds depth: best Mole Man story since FF1.
129-130: Retelling of earlier Frightful Four battles where females were the key (e.g. circa FF40)
(note the title of 130 Battleground Baxter Building, is a homage to an earlier classic)
131-132: Inhumans. Old characters (new, forgettable villain), but adds depth. Excellent character development and historic last panel.
133: Thing v Thundra. Old characters, but another memorable issue that adds depth. Around this time Sue leaves Reed!
134-5: Combines old Dragon Man and Gideon stories in an effective way (and note Dorrie Evans!).
136-7: old characters (Hulk's Shaper of Worlds), but in a creative way. The first creativity!
Conclusion:
Note that half the stories merely retell old stories in slightly different ways, and each retelling either takes about twice as long as the original telling. or tells half as much. Time is still passing, depth is being added, and there is still creativity, but at a slower rate than before.
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1968: the year Marvel sold out

But give 'em a break - it seemed like a good idea at the time.
FF 61 Enter The Story - logo
 
best comics
value_____________
value: sales
science__________
science: tech
science: examples
marvel___________
marvel 1960s
marvel 1968
marvel universe
time_____________
time: examples
time: objections
time: continuity
4________________
4: lockjaw
badtime__________
What happened in 1968

1. The "Marvel Explosion:" many more titles published.
2. Jack Kirby decided to leave (he didn't actually resign until 1970)
3. Marvel saw significant income from merchandising.
4. Real time comics began to end.
5. Early 1969: prices rose by 25% (and by another 33% in 1970).
6. Marvel was sold to a corporate raider.

In short, Marvel shifted its focus: from growing readership to monetizing brands.

Sales fell

Up until 1968, sales increased year after year. After 1968, sales declined year after year.

Roy Thomas: In 1968 Marvel expanded. Every super-hero had his own title - Iron Man, Sub-mariner, Captain America.
Stan Lee: I was drunk with power.
Roy Thomas: And soon after that, there was a downturn in sales in general. Do you think there was an over-expansion?

Stan Lee: ...They go up, they go down. It's hard for me to remember specifically any particular event or why it happened."
- from Comic Book Artist Collection by Job B Cooke, Neal Adams, page 112

The "industry trends" theory

A common explanation for the reducing sales in 1968 is that all comics were losing readers at the time. But there had been a steady decline since the late 1940s: the 1960s was no different. Between 1961 and 1967 Marvel proved that all you need to buck the trend is realism and amazing stories. In 1968 Marvel changed dramatically, asking readers for more and giving less in return. Is it a coincidence that Marvel sales then fell?

The "unrepeatable luck" theory

Marvel has been milking these brands for so long that it's come to look back on the 1960s as a fluke, as unrepeatable luck. But as Jack Kirby said, this was not luck, it was hard work and a passion for realism. Marvel can repeat this luck any time it wants: every time Marvel has tried realism, sales have gone up and new iconic brands created.

Merchandising

Merchandising began to grow in 1966 and 1967, and by 1968 Marvel could see its attraction: licensing deals are one hundred percent guaranteed no-risk profit! Today Marvel is almost entirely a brand management company with only a small percentage of its money coming from comics. The comics exist to serve the brands, and so character development - the basis of a realistic story - is against Marvel's interests.
Less effort on each book

After 1967 the best talent was spread ever thinner. This is often the case in later years as well: fewer titles means higher quality. Take Spider-Man for example:

"It's funny, because looking back at how I have evaluated each of the spider-writers, there seems to be a consistent thread. In the time periods in which there is only one, or two main Spider-Man titles, the writing is definitely of higher quality. As the spider-verse expands, AND the crossover and other event driven gimmicks take over – the quality drops precipitously."

This is very clearly seen in 1968, and not just because the number of comics increased:

Less of a shared universe

When Marvel had only eight comics it was easy for one person (e.g. Stan) to be intimately involved in them all. You got a feeling that each story really mattered, and the shared Marvel Universe was naturally seamless. With more books, more details were forgotten or ignored.

Stan was less involved in the stories.

"Roy Thomas: Everyone's heard tales of you physically playing out stories, jumping on tables, and acting out 'Thor' stories.
Stan Lee: I used to enjoy doing that. .. writing at the typewriter, hour after hour, got kind of boring. I would do whatever I could to jazz it up."
- Comic Book Artist Collection by Job B Cooke, Neal Adams, page 111
With more comics, Stan took more time at editing and managing and less time at the typewriter. So he didn't know the stories as closely and never needed to act them out. With so many more stories to cover, Stan wasn't even aware of what was happening in each comic any more. The late 1960s Fantastic Four for example were almost one hundred percent plotted by Jack Kirby, and Stan just added dialog. The results could be amazing, but only if you had the world's greatest comic creator on the job. Then Jack left.

The best people leave

Steve Ditko had left in 1966, over his refusal to compromise with the realism of Spider-man. Jack Kirby started to plan his exit in 1968. In that year Mark Evanier remembers seeing sketches of the New Gods. Kirby stayed on at Marvel for another two years, but he was withholding his best material, and it showed. 

Why did he leave? Because Marvel was only interested in milking his brands, and treated him as just another employee. Marvel had the brands, they no longer needed the man who created them.

Replacements are inexperienced or disinterested

The Marvel Explosion, and the loss of Ditko and Kirby, meant new talent had to be brought in. The new talent fulfilled the corporate need: to milk the brands. To keep writing the same kinds of stories . They had no reason to want new ideas.

Roy Thomas
Most of the new writing was by Roy Thomas. But he was a fan who came straight from school (except for a fewer months as an English teacher). He was, and still is a great guy, a real fan's fan, but he lacked experience.

John Buscema

John Buscema was a great artist, but didn't care at all about superheroes. He was the last person in Earth to be passionate about making them believable.
"I'm not interested in any super-heroes. Some of the Conan books I enjoyed. I'm sorry I didn't ink more of them, but at the time I wasn't interested. All I was interested in was how much I could make today. .. Anything with super-heroes, I'm not interested." (source)

John Romita
John Romita was another great artist, but had no interest in new ideas.
"It had become very hard for me to come up with new ideas.... So I said, 'If I do any comics ... I'll do inking only...." (source)
His Spider-Man looked much classier than Ditko's, but also lacked the realism.

Gil Kane
Gil Kane was another great artist who lacked any interest in plots.
Gerry Conway explains:
"after doing a few stories with him in my usual loosely plotted style, I began giving him tighter plots, indicating where the story had to be by such-and-such a page. He seemed to prefer this." (source)
His art did have a good realistic edge, but he was so good that Stan kept him mainly on covers, so we never saw a consistent Gil Kane influenced story.

Gene Colan
Colan was perhaps Marvel's best artist, when space was not an issue, but like Buscema he wasn't that interested in the plot or in superheroes generally.
"Colan would from time to time rewrite plot points so that they were easier to draw. Colan made no secret of his preference to work on horror titles." (source)


The heir to Kirby and Ditko was driven away

Creative geniuses are rare but they do appear every few years. Once such was Jim Steranko. When modern fans look back at ground breaking comics that still bring gasps of amazement, they remember Jack Kirby, Steve Ditko, and Jim Steranko. Jim appeared at just the right time: hitting his stride in 1968 as the others were leaving.

Like Ditko, Steranko refused to compromise his vision, and so he was driven away. (Details; more details.) 

Like Kirby and Ditko, Steranko held (and holds) passionate views of honor and truth. Like Kirby he was a fighter with a wealth of experience and interests to draw from. He provided "incredible exploits and down to Earth realism" that amazed and attracted fans, but that apparently wasn't enough. Marvel was only interested in milking old brands. New ideas were not a high priority.

"Steranko had realism? Really"

We may not associate Steranko with realism, but go back and look at his comics. Steranko's superpowered people were good, but nothing special. But his ordinary people have never been equaled: Captain America, Nick Fury, or his horror and romance comics. Steranko could make ordinary people seem amazing in a way that nobody else could! His clean lines, bold colors, facial expressions, use of ordinary objects, use of photos, etc., made everything seem hyper-real.
The price doubled

The Marvel Explosion saw 8 books increased to 12. Soon after, the 12c price increased to 15c. The price of being a dedicated Marvel fan almost doubled, from 96c to $1.80.

This may not seem a big deal, but since time immemorial, comics had been cheap. For more than a quarter century, including the entire golden age, comics had been a dime. For the silver age, comics were 12c. That 20% price increase was still resented, and helped destroy the biggest selling comic of the 1950s. Now a further 25% price increase, soon to be followed by an additional 33% price increase to 20c, meant the whole foundation of comics - good value for money - was being undermined.

"Why does price matter? I though only realism mattered?"

The value of comics is increased by realism and excitement (see the formula given in FF9.) That value is measured in dollars and cents. A comic that may be great value for 12c may be poor value at 15c. It's not rocket science.

Fewer panels per page

Not only did comics cost more, but you got less for your money on each page:

"By the late ‘60s, Kirby had largely switched over to a looser, more action-oriented style, incorporating more full-page panels and three- to four-panel pages. Rumor has it he asked for a raise, was refused, and Stan told him to draw fewer panels instead. Another possible factor: In the mid-‘60s, the American comics industry switched to a smaller standard original art size. Before, artists had usually drawn at roughly twice printed size; the new standard was only about 1 1/2 times printed size. This led to less detailed pages but encouraged a more intimate look -- and fewer panels. ...  Stan Lee printed a few complaints about this change of pace -- or, rather, pacing -- in the FF letter columns of the time. But for the most part, fans liked it because Kirby’s storytelling abilities were at their peak." - from "A THOUSAND FLOWERS: Comics, Pop Culture, and the World Outside," Installment 27, by Stuart Moore, 09-23-2003 on Newsarama.com

Perhaps the most famous example of this, often referred to at the time by Stan Lee,  is where Gene Colan took an entire page to show a hand on a doorknob. It was beautiful picture - Colan is a stunning artist, the best Marvel had, by a long way, if space was not an issue. But space was an issue, and this meant less story for the customer.

Stretching stories

With less on each page, more stories had to stretch out. Fans noticed and they complained. Stan defended himself on the splash page of FF61, and later promised to stop all multi part stories. But multi part stories were never the problem. Fans love multi part stories, as long as each book gives value for money, and Stan was forced to bring them back. What fans hate is stretched out stories, where you pay for "a book length bombshell" but only get a chapter.
Selling out to a corporate raider: 1968 and 1996

Every comic fan knows what happened in the 1990s: Marvel was sold to Ron Perelman, and all he wanted was short term profit. So prices went up, Marvel was told to sell more comics, and after a brief investment bubble sales collapsed and Marvel went bankrupt.

Almost the same thing happened in 1968, when Marvel was sold to Martin S. Ackerman,
"a flamboyant ...lawyer and businessman whose career was in mergers, acquisitions, financial workouts and banking. Business Week magazine described him as 'a razzle-dazzle financial operator. In 1968 he became president of Curtis Publishing by lending $5 million through the Perfect Film and Chemical Corporation, a conglomerate that he had built.'" (source)

Ackerman then famously halved the circulation of the Ladies Home Journal, a magazine started by Benjamin Franklin, and selling six million copies. After gutting it he sold it, and it closed down. We don't know exactly what he did at Marvel, but 1968 looks eerily familiar to Ron Perelman's early 1990s:

* The number of books increases dramatically.
* The price of each book increases dramatically.
* The quality of each book goes down.
* Sales go down

Ackerman then quit Curtis and Perfect Film in 1969. The dates may be a coincidence of course: the Marvel Explosion happened a few months before the company was sold. Perhaps Martin Goodman (Stan's boss, who made all the decisions regarding pricing, pages, titles, etc.) could see a potential sale on the horizon and wanted to make the company look more valuable? As for the price rise, every price rise in history has been called unavoidable and regrettable. Who knows? Whatever the whole story, the actions and results were the same.


Marvel was not dead... yet

Marvel did not die in 1968. The next few decades included many great and memorable stories, including many excellent near-real-time stories. But the focus on brand management led to Marvel Time. Marvel Time crippled the ability to create realistic stories. The Marvel Universe stretched thinner and thinner and eventually snapped in 1991. But it was great while it lasted.



Conclusion

In conclusion, Marvel ceased to be the house of ideas, and became a brand management company. They literally sold out in 1968.

Which was the year I was born.

Not that I'm bitter or anything. :)
Marvel comics burst onto the scene in 1961...

constantly creating major new characters...

sales increasing year on year...

until 1968.
 
1968: the end of the Silver Age

The glory days of Marvel ad DC are often referred to as the Silver Age.

How and when did the Silver Age end? Comic fans disagree, but the real experts are the people behind the Silver Age Marvel Comics Cover Index (SAMCCI). The SAMCCI Reviews Section offers an in-depth analysis, issue by issue, of Silver Age Marvel. The era is divided into four parts: (1) The Early, Formative Years, (2)  The Years of Consolidation, (3) The Grandiose Years, and (4) The Twilight Years. So here's the sixty four thousand dollar question, when did the Silver Age end?

"Despite the temptation to use Jack Kirby's departure from the company as marking the end of the Grandiose Years, the fact is, no era can be demarcated by a single event.  In fact, it's the contention here that Marvel's exit from the Silver Age began almost two full years before Kirby left."

"Almost two full years before Kirby left" takes us to the changes of 1968. And I agree that the end of the Silver Age was a process, not a single event. But as they state, Marvel's exit from the silver age began in 1968. This is what changed, according to the SAMCCI reviews:

1. Less interest from writers. The new generation of writers and artists "steered their most creative energies away from the books that had become the bedrock of the line."

2. Reusing ideas. "Elements such as characterization and realism began to fade and humor became stale; formula trumped originality."  See the SAMCCI reviews of FF 84, 85 and 87 for examples.

3. Less care over continuity. "There was some internal consistency with past events, but little reference to the wider Marvel Universe" with stories "Riddled with more inconsistencies than readers this late in the silver age could be expected to swallow without abandoning their suspension of disbelief"

4. Loss of institutional memory. The authors refer to "the eventual loss of the institutional memory of the industry's older professionals." The older professionals remember a time when superheroes did not dominate, when there was no fan base, when they had to constantly change to attract new readers.

5. The end of optimism. "The high-flown, optimistic language of Stan Lee that not only captured the spirit of the 1960s, but caught the imagination of a generation of readers coupled by the soaring visions of Jack Kirby, Steve Ditko, Don Heck, Jim Steranko, John Buscema & Gene Colan, had all but vanished by the mid-1970s." I especially miss this.

The common thread, the villain of the tale, is Marvel Time:

1. Less interest from writers.
Marvel Time limits what a writer can do. So naturally writers feel limited by these books.

2. Reusing ideas.
See above.

3. Less care about continuity.
Marvel Time allows the same villains to crop up again and again. This makes continuity both confusing and dull, so nobody cares.

4. Loss of institutional memory.
Nobody remembers a time when you had to fight for readers, because Marvel Time allows you to offer the same product to the same fans, forever.

5. The end of optimism.
Marvel Time means nothing really changes, so there is nothing to look forward to.