This was one of the four superhero movies that I really enjoyed (the other three were Watchmen, Iron Man and X-Men: First Class). I only had two major issues with the movie. First, I like Scarlett Johansson as much as the next guy, but she was completely miscast as Black Widow. She's a babe, not an action heroine. She only had three facial expressions throughout the entire flick, and all of them said "I'm such a cute babe" instead of "I'm a total badass".
Second, Captain America's outfit In C.A. - The First Avenger, they managed to make him look halfway decent. I didn't care much for the plot, but at least he didn't look like a total idiot in that outfit. So why did Whedon have to dress him up in that ridiculous blue romper suit? I mean, it's bad enough that the guy carries a shield and calls himself Captain America. At least give him some kind of armored combat suit, like the one he wore in the 2011 movie, and perhaps a side arm.
Other than that, the movie was as good as superhero flicks can ever get. Tom Hiddleston was an inspired choice as Loki and totally stole the show, which is not easy to do in a movie with Samuel L. Jackson and Robert Downey Jr. He was the best supervillain since Kevin Spacey as Lex Luthor, imho.