Minor spoilers ahead? I guess....
Saw it last night, and while I think the Nolan movies are GREAT films based on a comic book character, The Avengers is one of the best "super-hero" movies I think I've ever seen. It's certainly nothing new or revolutionary, you have the requisite, heroes butting heads, the one-liner quips (some of which weren't 100% predictable) and the unifying moment when they all have to come together...but the pieces just work together.
As great of a character as Tony is as the self-centered narcissist genius, I am always a fan of shades of grey, and what humanizing was attempted in Iron Man 2 was done miles better here in a single scene with Banner.
Loki feels like a major threat throughout the movie, even when captured fairly early, his presence is ALWAYS felt. The action scenes are superb. Over the top while just falling on the good side of being cheezy. The only down side is that, after serving their role early on (and serving it well) Hawkeye and Black Widow feel way out of place at the climax. Pistols just don't cut it.
And Mark Ruffalo's Banner/Hulk is the high point of the movie.
Random observation: The phrase "Avengers Assemble" (or the word "assemble" for that matter) appears nowhere in the film....at least as far as a I caught.