Hoping to avoid spoilers, but read at your own risk.
The moment The Avengers was announced, I knew I was going to it. A movie with Iron Man, the Hulk, Thor, and Captain America? How could I not? But at the same time, I was a little leery because the film would have a lot of strong, well-loved characters who'd demand screen time and I haven't seen all that many superhero ensemble films-- The Fantastic Four comes to mind, and that was abysmal.
I was glad to hear that Joss Whedon would be helming the film, though. I'm not a massive Whedon fan, but I've enjoyed his work and trust his writing and directing, and I know he's a geek and a fanboy. I figured he'd do the movie and characters justice, if anyone would.
I was torn by the trailers. They did a great job of inspiring confidence that the film would look awesome and treat the characters appropriately, and they were wonderful at conveying the general mood and plot without actually giving details. But at the same time, they had Thor fighting other Avengers, didn't show very much of the Hulk, gave the impression that there wouldn't be much substance (just battles), and had some valid complaints lobbed at it re: the fact that Black Widow has a tiny gun. I also wasn't sure how I felt about not entirely knowing the villain ahead of time.
The movie is and isn't anything like the trailers. We get the awesome battles and the snark, but we also get a tight, thinking-person story that keeps you guessing, and strong and believable character dynamics. It's just as thrilling and inspiring of nerdy glee as you'd expect. The science looked and sounded plausible. One of my bigger worries was that the Avengers would be introduced in a single scene, but I shouldn't have: Whedon spread the characters out so we'd get to know them one by one, and did it in such a way that the introductions worked with the story, rather than feeling forced. There were some excellent gags that had everyone in the theatre laughing. And Whedon even managed to bring out the personalities of lesser Marvel movie-verse characters Hawkeye and Black Widow, and to make me want an origin movie for the two of them. While there wasn't a whole lot of character arc material, there was some, and I definitely like Ruffalo's portrayal of the Hulk (though I haven't seen the other recentish Hulk movies).
One thing I've been noticing more lately is the appearances and agency of women in fiction. I blame the internet. Whedon did a good job, in my unschooled-in-formal-feminist-criticism opinion—that one line that has online feminist fandom talking aside. Natasha Romanov/Black Widow comes off as a realistic woman, with friends and fears and a fantastic ability to beat people up. She has action and agency and isn't a sexual object. Maria Hill is equally awesome, though we get less of her personality and more of her second-in-commandness. I don't remember anyone treating her differently because of her sex. And there are bit parts that could easily have gone to men, but went to women, which was nice to see. The Avengers doesn't pass the Bechdel Test, but that's not the be-all-and-end-all of feminist critique for me.
A couple quibbles: Fury was great, but largely in the background (which is probably fine, it's The Avengers not Nick Fury). We saw him making decisions and saying they were right, without any proof he could make good calls, but that might've taken away some of the suspense, so that's fine with me in the end too. Still, Fury's an interesting character and I'd like him to look less like a figurehead.
I also felt that Loki was a little out of character with his scheme, but believable enough. I had more trouble with him willingly subordinating himself to another person. Loki definitely makes a good villain, though, and I hope to see more of him in other films. He plays long games and keeps people guessing, and I like that.
Quibbles aside, I really liked the film. Spent-2.5-hours-in-a-theatre-grinning-my-head-off liked the film.Going-to-see-it-a-second-time-on-the-big-screen liked the film, which doesn't happen very often. It's fun, it has action and explosions, it has superheroes and some of my favourite characters to boot, and it has plot! That doesn't strictly follow the origin story model! That I couldn't predict!
Go see it, if you haven't already. Or see it again, if you have.