One of the reasons that the Coulson death worked so well (and really HAD to happen) was because the audience had come to know him so well. I can't currently think of anyone I know that didn't like that character. His death in The Avengers was not just an emotional turning point for the characters, but for the audience as well. I just don't think we've built that kind of attachment to Hawkeye yet. The same can be said for Maria Hill. I would agree with what I saw someone else on here state: Iron Man is actually a logical choice. However, I think that if they were to do that, it would have to come in The Avengers 3 (assuming that we get three movies). Iron Man is, from my understanding, by far the fan-favorite character among general audiences (by 'general audiences' I mean people who have little to no knowledge of these characters outside of the films). I think that killing him off in the second movie would alienate a lot of movie-goers before the third.