I'm so glad this thread got made, because there's alot more about it that I want to discuss than just my general thoughts that I posted in the movies forum.
The two things that really made this movie for me were the character interactions and the surprisingly witty humor.
Everyone has had a fair bit of character development in the previous movies leading up the The Avengers, but here we get to see them bounce off each other and develop together. It all seemed very natural and I liked that even though they didn't get along to well to begin with, they didn't let their differences get in the way during an emergency like so many other fictional stories do to play up drama.
The humor was great, and considering Joss Whedon wrote the script, that really shouldn't come as a surprise. But what can I say, when I went into the movie theater I was expecting to see alot of great action and a bit of trademark Stark Snark. I wasn't expecting to be laughing out loud just as often as I was taken in by a good action scene. Everyone had their funny moments. I'll spoiler tag a few here for those who haven't seen the movie yet:
Loki getting the crap pounded out of him while the Hulk throws him around like a rag doll. The action is only topped by his following line. "Puny God."
An earlier exchange as Captain America chases after Thor-
Natasha: Be careful, they're basically gods.
Cap': There's only one God, ma'm, and I'm pretty sure He doesn't dress like that.
Tony Stark pretending to direct the ship. Part of it has become my sig quote-
Tony: Hoist the main sail, raise the mizzen mast. You, take us about. That man there is playing Galga. He thought we wouldn't notice, but we did. I... *covers one eye* How does Fury see all this at once?
(Part of the reason I like it so much is that I love Galga, it's my favorite old-school arcade game)
Thor sort of defending Loki-
Thor: He is of Asguard, and my brother!
Fury: He killed 80 people in two days.
Thor: He's adopted.
Anyway, sorry for the uber-post, I just really enjoyed this movie. Only complaint? Wish we could have learned more about Hawk-eye. He was Brickin' Awesome through the whole movie
Both brainwashed and good but I wish we could have learned more about his character in this universe. Here's hoping for a Hawk-eye movie in the future.