In this modern era of blockbuster cinema, where movies based on comic book superheroes, The Avengers still manages to reign supreme above the rest, at least in terms of box office. This is perhaps based upon the shrewd way Marvel built up to this movie. It served as the culimination of a process beginning with the first Iron Man and building from there. Many of these characters were already established, bits of information was dealt out in post-credit sequences, and in the end a group of dispirate superheroes were assembled.
The Avengers wastes no time getting down to business, even if for an action movie there is a lot dialogue and talking scenes. However, fans of action need not worry, as there are plenty of fights as well, and not just the good guys versus the bad guys either. In the broadest of senses, the movie is about Iron Man, Captain America, Thor, The Incredible Hulk, Black Widow, and Hawkeye joining together to take on Loki and his army of aliens. The first three have all had their own films, as has Hulk, but with two different actors, neither of which are Mark Ruffalo, who took over the role of Bruce Banner for this film. Black Widow showed up in Iron Man 2, while Hawkeye, played by Jeremy Renner, had a blink-and-you-could-miss-it cameo in Thor.
Loki makes sense as the main villian in the movie, as not only was he established in Thor, but he's also quite powerful, as some sort of super powerful space being or something. It probably should be pointed out that having seen all the movies leading up to The Avengers would certainly be beneficial, but it is not essential to enjoying this film. You may not have a full sense of the characters, but you will learn enough just from within the parameters of this movie. There is a self-contained plot here that can mostly sustain itself without outside knowledge, which is important, as asking folks to see a handful of movies just to understand this seemingly stand-alone film would be a bit much.
It also should be said that the logistics of this world are not quite realistics, what with Thor and Hulk hanging around and such. However, if you are interested in watching this movie, you presumably know at least something about the characters, and are likely able to accept such matters. Sometimes it is a bit too convenient, or a bit much, particularly when it comes to Loki's powers, but it is only a minor issue.
Overall, The Avengers remains a good action movie. Joss Whedon, who both wrote and directed it, has a certain style, which is fairly evident in this film, and it serves the characters well. He does get a bit too quippy at times, but most of his jokes hit, although having Robert Downey Jr. around to deliver most of them helps. Downey is the highlight of the movie, as he has really found his stride portraying Tony Stark, although he was successful at it since the first Iron Man, which is part of the reason why this movie even exists. Ruffalo plays a good Banner, while, alas, Renner is left with little to do as Hawkeye. There are some good, exciting action sequences and fight scenes, many of which are turned up to eleven in terms of damage and physics. The ending is also really good, perhaps the strongest aspect of the entire movie. This refers to the pre-credits conclusion, not the two scenes within the credits themselves. The first of those seems to tease future developments, only fans of Marvel comics will likely enjoy it, while the second is quite funny but completely tagged on.
Clearly, The Avengers was a huge success, one that has allowed Marvel to continue cranking out new movies for these characters, not to mention whole new characters. However, it also succeeds as a movie. It's a lot "funner" than Christoper Nolan's Batman movies, which is too say it doesn't take itself as seriously and is more content to crack jokes and bring the action, although it doesn't completely skimp on the emotional heft, even if it may not be as good as those films.