For completion's sake I suggest you check out "The Incredible Hulk". You don't really need to see the earlier movie from Ang Lee with Eric Bana and Nick Nolte, but there's a certain level of continuity between Iron Man, The Incredible Hulk and Avengers Assemble. (There's also a slight continuity error which was corrected by a short film called "The Consultant" on the lead up to the release of "Avengers Assemble".)