This film brings the protagonists of the following Marvel movies together: Iron Man, Thor, Captain America: The First Avenger, The Incredible Hulk. It also features characters shown in Iron Man 2, so you should watch those movies first.
After being banished from Asgard, Loki, Thor's brother makes a pact with a powerful alien entity. The entity would give him an army and Loki would give them the Tesseract, the powerful artifact Hydra used in Captain America. Loki steals the Tesseract from a S.H.I.E.L.D base and takes control of the minds of Dr. Erik Selvig and Agent Clint Barton (the Hawkeye from the comics). In order to retrieve the Tesseract and deal with Loki, director Nick Fury of S.H.I.E.L.D decides to reactivate the Avengers Initiative. He recruits Captain America and sends Agent Phil Coulson to recruit Tony Stark (Iron Man), and Agent Natasha Romanoff (the Black Widow from the comics), to recruit Dr. Bruce Banner (the Hulk). Thor joins later when he comes to Earth to take the Tesseract back to Asgard and try to reason with Loki.
The movie is great. I can easily say that it's the best superhero movie ever made (Nolan's Batman isn't really a superhero movie IMO). Marvel did a great job. First, they did the heroes' solo films to introduce them and their universe. And then they did a crossover. Joss Whedon handled the direction very well. Since he didn't have to introduce the characters and the setting, he went straight for the plot and the crossover. Now, Hawkeye wasn't really intorduced before, so making Loki control his mind to fight the other heroes worked well to show what he could do. When it was time for him to fight along the others, we already knew what he could do.
Whedon gave a lesson on how to make a crossover. He gave each hero enough time on the spotlight and even managed to throw some development for the Black Widow, since she didn't have her own movie (and probably won't). He also followed the "recipe" well. The heroes at first fight one another, then try to work together but fail and then become a real team to battle evil. The text is also good and very funny. The characters are well written and the movie did a great job in adapting them from the comics.
The action scenes were great and he managed to show the power level of each heroes. While Iron Man, Huk and Thor were doing battle agains very strong foes, the Widow, the Captain and Hawkeye were each contributing by fighting footsoldiers, helping civilians and using the cover of the big guns to do important things. It was nice to see how he used Hawkeye's talent and how he made the Captain a good leader in battle. The desing idea for Hawkeyes bow was also very cool.
The acting was also good, with Downey Jr. doing an awesome Tony Stark and Tom Hiddleston doing Loki so well that it looks like he was born for the role. I still have problems with Scarlet Johansson as the Widow, because she is petit, but I can't complain of her acting. Mark Ruffalo is the newcomer and he made a very good Banner. The other actors pretty much did the same good job they did in previous movies.
This movie was awesome and it changed the paradigm of superhero movies.
Veredict: Must watch.