Eric Newsom sent me this link to the text of an interview that Ditko did in 1968 for the fanzine, Marvel Main. If you look at the work he was doing for Marvel just before he left and then you read his words from this interview, you can get a pretty good idea of the creative tensions that were going on in the Lee-Kirby relationship. The company dynamic had evolved into offering a diet of "heroes" who were either flawed or uncertain of their own heroism and values. That's not the way Ditko saw the world.
Also, Josh Wendell just wrote me to ask…
I'd heard Ditko quit Spider-Man because of the storyline about the Green Goblin who was about to be unmasked. Ditko wanted it to be one person and Stan wanted it to be someone else. Is this not true?
It may be that they differed on that plot point…although you also have to remember that the two men weren't speaking by this point. Ditko was plotting and pencilling Spider-Man, then handing it in with notes to Stan's Production Manager, Sol Brodsky. Stan then dialogued the material and it was lettered, then returned to Ditko…who increasingly disagreed with the dialogue Stan had scripted and the changes he'd made in what Ditko regarded as "his" stories.
But come on. You don't quit a company that's employed you for years because you want the Mystery Villain to be one guy and your collaborator wants it to be someone else. At most, that might be a Last Straw that caused you to ask off that particular book. Ditko left Marvel completely and either said or gave folks there the impression that he was never coming back. I'm not taking any sides in who was right or who was wrong…but obviously, Lee and Ditko weren't getting along well before they got near the Green Goblin unmasking storyline. More than a year previously, Ditko demanded and got a plotting credit, and probably some writing money.
And as the friction grew, Stan stuck Spider-Man into the Daredevil comic so that he could "audition" its artist, John Romita, in case he needed to replace Ditko. It had all been building for some time before Ditko quit.
One other thing I should have mentioned: A few months before Ditko announced his resignation, Marvel made the deal for the Marvel Super-Heroes cartoon show, the first time those characters were transferred to another, more lucrative medium. The show went on the air with segments of five Marvel heroes (Iron Man, Thor, Sub-Mariner, Hulk and Captain America) "animated" — I'm using that verb loosely — right off the comics that had been drawn by Jack Kirby, Don Heck, Ditko and the others. Ditko didn't mention this to me or to anyone I know who's discussed those days with him…but Kirby and Heck were sure rankled that their poorly-paid comic book work was being transferred to television without any additional compensation. It would be surprising if Ditko wasn't.
It's also not widely known but when the deal for that show was made, Sub-Mariner was not included and Spider-Man was. The original sales presentation film (a bit of which I showed at the Comic-Con in San Diego last year) included Spider-Man. He was taken out of it because a more lucrative offer for a network Spider-Man cartoon series seemed likely…and indeed, such a series was on ABC the following season. In any case, at the time Ditko left Marvel, he was well aware that his co-creation had been sold for a TV show and that there would likely be a flood of Spider-Man toys and merchandise, and that he wouldn't be sharing in that windfall. I can't think of another freelancer in comics who wasn't upset when he found himself in that situation, even if some of them chose to not go public with their feelings.
So is it really that huge a mystery as to why Steve Ditko quit Spider-Man?