1. Natasha Romanoff. Is anyone surprised that this is my number one? I love how her vulnerabilities make her stronger. I love how she tricked the trickster. I love that when I thought she was in a damsel in distress scene, she was totally pissed off at her "rescuer." I love that she had a little freak-out by herself after she escaped from the Hulk. I love that her BFF is a man. I love that amid all these superheroes, she doesn't have any superpowers, but she holds her own. I love her achetypally male tortured-past-longing-for-redemption back story. I love that she doesn't cry over fallen regimes. I love her ass.
2. Plot structure. Okay, so the first thirty mintues are pretty slow. Tesseract, blah blah blah, exposition, blah blah blah, physics, blah blah blah... But after that, the film is very tightly paced and designed with an eye to making the climax credible to the audience. They set up already that Stark Tower is a self-sustaining power source. Hulk's rampage on the ship established his character, but also showed his strength. Tony and Captain America's (Steve? Bucky? I get confused...) argument about self-sacrifice set up the climatic scene nicely. There were lots of scenes like this, which I will no doubt appreciate more on my second, third, fifth, and eleventh viewings...
3. Language. Joss Whedon writes terrific dialogue. I think we all know that. In this film, I loved how the characters literally and metaphorically do not speak the same language. On the metaphorical level, their personalities and perspectives are too different for them to connect, and that's mirrored by the fact that they literally do not speak the same language. Steve can't understand anyone's pop culture references or scientific dialogue. Thor talks about creatures in Asgard that no one else has ever seen. Bruce talks about science on a level that other human beings can't understand.
4. Fight scenes. This is not something I generally appreciate in a movie, but I truly enjoyed them here. I think Joss Whedon did something very intelligent in crafting the story. It would have been very easy for the Avengers to have teamed up against a group of super villains, sort of like how the earlier X-Men films were set up. Here, though, they have an external villain that appears primarily at the end of the film. This means we actually get to see the Avengers fight each other, thus answering every fanboy's favorite question: "if _______ got into a fight with _______, who would win?"
5. Vulnerabilities. I started wanting to see this film after I read an NY Times interview where Joss Whedon said "Black Hawk Down" inspired him to tell a war story where the audience isn't certain the good guys will win. He went on to say that feeling of vulnerability is crafted by creating conflicts between and within characters who are basically broken people. I think he delivered that admirably while at the same time creating physical obstacles that seem insurmountable. Like, just when you think Tony is going to fix the engine and everyone will be safe, the other engine blows out. Meanwhile, no one can come to the rescue because they're running from a rampaging Hulk. When the Avengers threaten the aliens, the aliens get smarter and attack them harder. The most scared I felt in the entire film was when Captain America got cut. Then, when victory seemed ineviable, a nuclear weapon is speeding toward Manhattan. The whole thing is very tightly paced, and even when lots of things are happening at once, it never feels extreme or overdone.