I think I'm the only one that didn't love this movie. Seeing this is the spoiler thread, I'll mention a few scenes that just didn't work for me:
1) Agent (Phil) Paulson's death: How doesn't the death of 80 people in two days get the Avengers together, but the death of Agent Paulson (at best a marginal character) does? While he was a constant in the other movies, he was never an important character. In this movie he just met Cap and Banner. While he knew Black Widow and Hawkeye, we never knew how well. He was a marginal character in the movie Thor. The only Avenger his death would have had an impact on was Tony Stark. The only reasons he was killed off was to make room for Agent Hill.
Loki: C'mon, he's a GOD. How can Hulk even grab him (remember, Thor gets suckered by an illusion of Loki earlier in the movie?), much less beat the crap out of him? Why do Selvig and Hawkeye just stand there and let Loki touch them with his staff?
Thor arriving back on earth: At the end of "Thor" he was unable to get back to earth, watching Jane Foster from afar, but in this movie he's able to get to earth to get Loki back.
Pepper Potts: Not sure why she was even in the movie. She just had a few scenes with a few throw away lines.
The aliens: Forgettable, overly designed, mindless killing machines. Just boring.
The argument between the Avengers in the lab: seemed forced, awkward. It was unclear why they were mad at each other. Was it the staff, was it because they found out Shield's plans for the cube? The fact viewers have to wonder about the reason is a problem.
Black Widow bargaining with Loki: Man, that was just hard to watch. It was far from convincing.
Did I hate the movie? No, but I really expected more.