I still remember seeing the first Avengers movie in theaters- after all, it was only three years ago. After I got out of the theater, my reaction was "Wow, I don't think I've ever seen a movie that I've liked that much before." I watched it again when it came out on DVD, and I noticed a major issue...
Before The Avengers, all we know about him is he works with SHIELD and has a bow and some arrows, and the only reason we knew that was because of Thor. Without even taking time to establish the character in The Avengers, bam! Loki's controlling his mind! And yes, later in the film we find out he worked with Black Widow and they're fairly close. But, at that point in the film, why do we care that Hawkeye wasn't being himself? After all, we didn't know him. A much better choice would've been Loki controlling Captain America- after all, even if you didn't see Captain America: The First Avenger, he's pretty well known. Of course, the issue continues in Age of Ultron. There are sopilers after this, if you still haven't seen AoU.
Unless you read comics or saw Days of Future Past, you don't have the slightest clue who this man is when he dashes on screen. So, at the end of the film when he dies, why should we care? After all, we know more about Hulk, Black Widow... Even Hawkeye! Now, of course Quicksilver, could be brought back, although lets be honest here, at some point that card is just overplayed.