I'll bet Ruffalo surprises us with his Banner. The one problem that Norton had is that he seemed to be an inherently angry, on-edge guy. Banner is supposed to be a mild-mannered scientist who becomes angry only after becoming Hulked. I think Ruffalo might be able to portray that dichotomy better. Don't get me wrong, I'd love to see Norton reprise the role but they could have found a worse replacement by far than Ruffalo.