For some reason Hulk never found his actor. In the past decade 3 actors attempted to play that role. All of them being very credible, none of them leaving that much of an impression.
Mark Ruffalo was fine, he wasn't impressive, but he wasn't underwhelming either.
Part of the reason I like Spider-Man more (1 and 2) as well as Batman (especially Begins) was the drama. The Avengers was an exciting and entertaining movie, but it was shallow, it didn't draw you in emotionally like the darker comic movies.
I wasn't left thinking after the movie, only revising the action scenes and that's fine - but I prefer substance.