Batman Begins rebooted the bombastic batman movies into a darker, quieter, more serious and far better series. Superman attempted to redo that series into a high-contrast heroic fantasy, and failed.
Iron Man made a similar attempt to be a bright action movie, and (IMO) did it very well. The other Marvel movies so far have been mostly good to great, but the most important two of them I think are Iron Man 2, and Avengers Assemble.
The Spiderman series is interesting of itself, but never really got over its own origin story. Hero movies have this weird obsession with the creation of the hero, and the first two Spiderman stories seemed to focus on the idea of becoming a hero, and when they got to the the point where he was fighting the evil of the week more than his own head, the film collapsed like an over-observed soufflé. Fantastic Four 2 did the same thing, to an extent.
Iron Man 2 started with Stark being Iron Man already. Being known for that, and already having the suit and the confidence, he went though the Wheel of Plot (Setup, Defeat, Man-Up, Win, Return to the world, Having learnt something) without needing to define the character at the start.
Avengers has two advantages to being an Origin Story. The first is that it doesn’t have to set up or introduce most of the main characters, but the second – and I think unique in Hero movies – it doesn’t need to set up the villain either. Tom Higgleson’s Loki from Thor was a strong and complicated character, and survived the denouncement at the end of his own movie. He’s even more awesome here than he was in Thor, which is a neat trick, and it gives me hope that the shooting-gallery of comic villains can work in a movie context, and that Marvel Studios is actually capable of producing comic book movies without rebooting into a new Origin Arc because a new writer/director wants to put his/her handprints into that set of concrete.
The writing is Whedonesque (obviously) and the characters stay true to their original movie characterisation while sifting slightly into the more quickfire style. It does a good job of setting up the strengths and weaknesses of all the main players both as heroes and as people, and actually pulls off all these massive characters working well as a team without losing the self-reliance from their own movies.
The action sequences range from slightly by-the-numbers though to absolute awesome, and there are crowning moments of awesome for everyone, even the complicated villain. Samuel L Jackson is recursive, given that he’s playing a character based on him, so he’s quite good. It’s good to see Colby Smoulders doing something other than HIMYM.
You need to know about Iron Man, you should probably have seen Captain America, but you probably need to have seen Thor before you watch Avengers. It does stand alone, but the story follows on from Cap and a lot of the characters will make more sense if you’ve seen Thor.
There’s one post-credits sequence halfway though (After the animated credits, before the white-on-black roll) but nothing at the end.