Was there a panel somewhere where red hulk said'i'm quitting the Avengers'? I'd like to see that. I know, or assume at least, Hickman did not want to use him, but I would have liked the transition to have been, you know, handled.
As for the others, I like Namor as a character and Winter soldier as Winter soldier. I was never a fan of Flash Venom or Dr Octopus as heroes. They belong in the convoluted Spider world. Strange and other magic folk are best left out of the avengers if possible. There is just no limit on what they can do which is a story killer. Iron Fist, I do not care for as an avenger, though as a solo player or hero for hire I do. Cage I really like. Bendis turned him around and the mighty book is keeping him good. I have mixed feelings about sandman and Hercules, though. I liked them, but I don't know they really fit in to a 2014 book, any more really than black knight...
Thunderbolts implied that he left because he had to have freedom to take down some of the projects that he had started before his change of character. Works for me, I suppose.