Well, I am sad to see Edward Norton go as Bruce Banner/ the Hulk, maybe Mark Ruffalo can pull of the Hulk like Lou Ferrigno, but I'm not confident in his ability to play Bruce Banner yet. It is a "we'll see how it goes" kind of process.
So long as Robert Downey, Jr. and Samuel L. Jackson remain on as Tony Stark/ Iron Man and Nick Fury, respectively, I think they can pull off the movies. That, and keeping Jon Favreau on as director.
Now it's a waiting game to see how the Thor movie turns out and gives us an indication as to the final product of the Avengers and Captain America films. I'm still in a 'meh' mood until I can see what Chris Evans and Chris Hemsworth can do as Captain America and Thor.
<crosses fingers> Here's hoping. -_-