If any reviewer has called it something like an "action-packed thrill ride", they're pretty much on the money. The movie starts with a bang and that sets the tone for the entire film. Then the individual members (Captain America, Iron Man, Thor, Black Widow, Hawkeye and the Hulk) band together and kick ass. That's it in a nut shell.
Fans have been waiting for this film for years now. As I said before, this represents a pretty smart and long-term plan by Marvel to release movies on individual characters that would lead into the team movie. This lets them get the origin stories out of the way in the previous films and also give fans a treat. It's probably the cinematic equivalent of a comic book series, where each movie has it's own story but there are bits and pieces that cross over to form a story arc. I'll admit it. It's pretty damn cool.
I was unsure of how Joss Whedon would handle the material, since he isn't really know for blockbuster movies, but then again, neither was Jon Favreau and he knocked it out of the park with Iron Man. I think Joss turns it up to 11 with the Avengers, and he has to considering all of the parts he has to deal with. Nobody gets the short shrift either. Iron Man gets a little more time than others, but everyone plays a part in the plan and they work together, shocking, as a team in order to repel the threat, complete with a plan and everything.
I think my only complaint is just a bit of casting that goes back to a previous installment, in that I don't think Chris Evans is a good Captain America. I think he's a good actor and I like him in most of the stuff he's in, but he just doesn't capture the look of Cap for me. Maybe it's because I imagine more of a Brad Pitt in the uniform. The costume may be part of the problem for that. While the other heroes have costumes that translate better from the page to the screen, Cap's ends up looking a bit off. Maybe it's the new one for the series as I thought it looked better in his own movie. Oh well, a minor complaint.
It's a great, fun film regardless, so check it out.