Re : AoU and the Marvel Universe in general. As somene who watches Agents of SHIELD, I'm glad to have Coulson, but I agree that it absolutely cheapens the whole emotional arc of Avengers, and that's a problem. Whedon knows it too, that's very likely the reason Coulson doesn't happer in Avengers : AoU, or any other post-Avengers Marvel movie for that matter. And this goes to a larger problem in the MCU which is that people, and especially good people, simply do not stay dead. I can't think of a single sympathetic character in the whole MCU that has died for good, which means that when you see someone die in a Marvel movie, you know it means nothing.
As for AoU specifically, it kinda worked for me because clearly the whole movie seemed to be building up to Hawkeye's death : we get to meet his family for God's sake. I wouldn't have been surprised if he had said something to the effect of it being his last mission before retirement, that's how heavily it was foreshadowed. So when Quicksilver dies instead of him, I initially liked because it was a subversion of expectations... but yeah, now that I think about it, there's just no way he's going to stay dead, and that annoys me to no end. Movies and comic books are different things, and maybe reviving people all the time works in the comics, but it certainly doesn't in the movies.