Re: Why Mark Ruffalo?
I liked both Norton and Ruffalo. They play Banner at pretty different stages in his life though. Norton plays him while he's still immensely tormented by what's inside him, while Ruffalo is in full control.
For a solo movie I think it's better to have Banner conflicted since that's what really makes that character interesting and that is what fills up the movie. In The Avengers he doesn't need to carry as much emotionally so there we can just go with the fun version of him "suiting up". Then again I guess that as soon as they stop using the weird message of "strength overcomes everything" Hulk works in every way.