Mark Ruffalo doesn't seem like the threatening type to me, but don't make him angry. You wouldn't like him when he's angry. Especially since there are major rumors going around that point to Mark as being the next Incredible Hulk.
Hulk ladies, not hunk. We already know you think he's a hunk. The soon to be shirtless Ruffalo is rumoured to be in talks to play the Incredible Hulk in the upcoming highly anticipated Avengers movie.
Why Ed Norton won't return is anyone's guess. Some people say he would love to return as the Hulk, others say that he wouldn't. Others point to a money problem and others say he doesn't want to be part of an ensemble cast. It's anyone's guess really. At this point no one knows for sure but Ed himself.
Anyway, I think Mark Ruffalo would make the perfect Bruce Banner don't you? He's so nice and non-threatening. As for being the Hulk, no problem. That part will probably be done in CGI so no worries. I think this is a case of perfect casting and I already want to line up to see Mark as the Hulk. No offense to Ed Norton.