I believe it's John Byrne who says that the straw that broke the camel's back and made him leave Marvel was when Jim Shooter called him in and told him that he should look at Shooter's "Secret Wars" as a perfect example of how to write a comic.
I was in high school when "Secret Wars" came out. It was terrible. Byrne and Miller and the other folks he ran off from Marvel were way more talented than Shooter even at their weakest.
When Byrne left Marvel and went over to DC, he did a mini-series for DC (called Legends, I think, it's been too long) and featured a comical villain that was basically Jim Shooter and lampooned the whole Beyonder/Secret Wars and then-current New Universe debacle.
If you start Googling, you'll find that:
Roy Thomas left Marvel for DC because Jim Shooter refused to renew the writer/editor part of his contract. Thomas' contract stated that he would edit the books he wrote. This had nothing to do with money, it was all about Shooter being a jerk and lying ...to the guy he says saved Marvel.
Jim Mooney left because Jim Shooter sent him a one-word thank you note for all of his service to Marvel. It said, "Retire."
Gene Colan left Marvel "because of Shooter." "He was difficult and threatening, and he doesn't have a good reputation." "He was making life too difficult for me. He made it too difficult for a lot of other people, as well."
Gil Kane left Marvel because "they had become another company. I think Shooter had come into the situation as being totalling in charge, and a new set of editors, and it wasn't quite... Roy was gone."
Steve Englehart left Marvel because Shooter "came in and decided that since he was Editor-in-Chief, he should be able to write whatever he wanted to write, and so he told me he was taking The Avengers away from me, and he told Steve Gerber he was taking The Defenders. Neither of us... I mean, we didn't like it, and basically that's why I left."
George Perez left Marvel because Marv Wolfman lured him to DC.
Marv Wolfman left Marvel because "things changed" when Jim Shooter took over.
The list goes on. Shooter was a micro-manager and that's poison to creatives.