Downey, Jr.'s contract runs out with Iron Man 3 and he hasn't yet been signed for The Avengers 2, but I guarantee he'll be part of it, even if Disney needs to pay him with a gold-plated F-16. He's the biggest name of the cast and he was the center of the marketing efforts for The Avengers.
He was also the only thing good about that movie, probably because he was the best actor of the lot.
I'm not a fan of super hero movies, however I've always enjoyed Iron Man, both in comics and the Downey movies. Don't know if it's something about the fact under the suit he's just a normal human being (I say normal lightly).
I really enjoyed Iron Man 1 but haven't seen 2 yet, is it any good?