By the time I got around to writing a reponse, The Avengers has already been a great success. The third highest-grossing movie of all time (I think if you ignore inflation) and it was somehow seen as both a risky and rewarding idea all at once. Five financially successful and separate movies all leading into one? Never been done before, but sure to make a ton of money. I enjoyed it. It was a well-made blockbuster, but I find myself not being able to think (or in this case write) all that much about it. In fact the more I try to, I don't necessarily find fault or error with it, but I realize that it isn't all that different from what you'd get from any other well-made action blockbuster that gets a summer release date.
I don't mean that as a criticism, far from it. That is more of just an observation. Joss Whedon, being the extremely intelligent writer-director that he is, has written characters that are fascinating enough that they can carry the whole film when they are all together no matter how much CGI-enhanced action sequences exist. At the end of the day, when they characters are all in a room, their dialogue being delivered by such talented performers could be a movie all its own. It works for everything from 12 Angry Men to Glengarry Glen Ross, so why not The Avengers?
The film doesn't go beyond that set-up, but it doesn't need to. The film is typical in that sense, but being disguised as a sci-fi film gives the whole movie a unique style to it. If I were to break it down into a really simple explanation, The Avengers takes whats good about the best kind of blockbusters and instead it just gives you more of "that". When the film enters its final half-hour of non-stop action, it doesn't feel like you are being overloaded with visual effects, but that you are instead just at the climax of a journey that is more fun than thoughtful.