Some of them are a real missed opportunity for me. Norton is one, but I cant blame Marvel for that one, he didnt help himself (though I think his beef about the TIH run time was right) and it was just better for both parties to go their seperate ways.
Whedon and Wright are both especially big missed opportunities. I know they got Whedon, but I dont think they let him of the leash enough with the Avengers movies. And thats why I find TA gets worse on repeat viewings and AOU had so many disappointing aspects to it. Neither are bad movies but with more freedom they may have been great. Wright is just a great director and so is a missed opportunity to me. Ant-Man might be good, but I will always feel we lost the better version of the movie when Wright left.
I too preferred Howard in the role of Rhodey, and really wish he had been kept, it wasnt his fault that deal was made where he got paid more than RDJ and Paltrow, and who in their right mind would turn it down?
Rourke I also understand, and I hope things work out with Portman and they write her character better in Thor 3.