In the trailer, it is pretty cool to see the Hulk rescue a falling Iron Man. But again -- how can that work? Maybe I simply don't get the Hulk or how he truly works.
As Jason noted, the Hulk was indeed one of the founding members of the original Avengers team. He is triggered by anger, but throughout his history he's fluctuated between being able to retain "control" in Hulk form. At the end of the Edward Norton film, you saw a brief scene of him "learning" how to control the Hulk.
Hawkeye had a brief cameo in Thor. He and Black Widow are, after all, second-tier Avengers. And with possible sequels or spin-offs, there's plenty of opportunity to get to spend more time with them.