Hulk and TIH are not disasters by any means, but both of them miss the mark a little in terms of totally encapsulating the Hulk.
Hulk played up the psychological aspect really well, but was a bit too dark and "artsy" (I use that term lightly) to really capture a general audience.
TIH was well rounded in terms of action and being a driven (if a little thin) story as well as really playing up the Hulk's destructive nature, but failed (imo) to capture the relationship between Banner and the Hulk....as well as having the Hulk be (for the most part) an inarticulate engine of destruction....which Hulk also had a problem with I suppose. Both films had a Banner character that, to me, seemed very run-down and depressed. Understandably so, of course.
The gold of the Hulk's character is not his ability to smash or destroy things (as awesome as it is), but it's the relationship between the two clashing personalities that really is the meat and potatoes of the whole thing. By having more of a hint at Hulk and Banner's relationship ("What if the other guy says no?"), having a likeable, less depressing Banner, and a Hulk that us both an engine of destruction AND an intelligent personality, Avengers seems to be on track for having a near perfect, if not totally perfect adaptation.