Don't get me wrong, I get it. For the sake of continuity, it is a bit jarring to see another actor play a role already established by another. Especially when the movie is part of a huge continuity involving other films. I understand why people might be upset if one looked at it from that perspective. But from the various blogs I've read regarding this, you'd think that it was some kind of travesty considering how fanboys are now out for Marvel's blood.
I make it well known that I'm not the biggest fan of 2008's Incredible Hulk. For a movie that's meant to be a much more action-packed than its predecessor, it was pretty boring, had terrible dialog, and Edward Norton was just completely uninteresting as Bruce Banner. Granted I could go on and on about why I prefer the Eric Bana film , but that's a blog entry for another day (provided I get enough motivation to write it). I would probably be a lot more understanding of this situation if Edward Norton turned out an RDJ like performance for Hulk. Maybe then, I'd agree that Norton is completely irreplaceable. However, there was absolutely nothing special about his performance (though that may be because Marvel practically raped his vision of what he wanted in the movie). And considering the fact that Mark Ruffalo (who I can totally see play a pretty good Banner) may be in talks of replacing Norton, I don't think I'll miss him all that much.
Then again, I don't really give a shit about the Avengers movie anyways. Iron Man 2 left a bad taste in my mouth, and if sacrificing character development and good stories for "continuity" is what is in store for upcoming marvel films leading up to The Avengers, then I think I may pass on the next few marvel movies.