[H]ow does the Hulk fit into this group? Isn't he a little too unpredictable? Isn't it rage (or pain or intensity) that triggers the Hulk? How can he actually work for good? How can he be a part of a team? Seems like that would be harder than other Avengers, even though it's Stark who says, "I don't play well with others."
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 has had absolutely nothing. How do we introduce the other characters and still get a cool plot with a decent bad guy?
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.
I have to say though -- I am looking forward to seeing how they tie all the movies, so far, together. I'm certain that is their point, and it seems like the work they put in should be a blast.