You may be aware that Marvel Comics in the past ten years have taken over the financing and making of their own movies (more precisely, movies with their characters*). With only a few sort-of-misfires like the Hulk movies, these have been generally very successful and more importantly, good flicks. It must be nice to have such a huge gamble pay off.
Avengers Assemble, and it was just really good fun. If you liked Iron Man, I think you will like Avengers. It was well done, a good story, often very funny, and even sometimes emotionally relevant, which may be the most difficult thing for a big summer action movie to be.
I was talked into watching Captain America first. I had not had any thought to see it, I'm not interested in War movies. But this was more like a Sci-fi/war/superhero movie, and it was fun. The visuals alone was worth it, many excellent machine and vehicle designs.
The Avengers movie was led up to, of course, by Iron Man, Thor, and Captain America. Despite having Natalie Portman in it, I thought Thor was not quite as strong as the others. Not sure why, maybe it had less humor, or maybe I thought the scenes in Asgaard did not seem convincing, it was not really clear if it was a "real" place or a spiritual realm, or what. But it was not a bad movie by any stretch, and if you want to get the whole background, it's worth seeing.
I hope the next Avengers movie will have that robot character, Vision, especially if in the female variety. And I hope it'll have the "Wondrous Wasp". That'll be a nice challenge, having such a tiny character. (They can leave out Ant Man for all I care.) I mention this because if Avengers had a fault, it was too few women. It had Scarlett and a tiny bit of Gwyneth, and then that goooorgeous woman from How I Met Your Mother, but she really didn't do much. ... Ah, her name is, believe it or not, Jacoba Francisca Maria "Cobie" Smulders. She does smolder. Unreal.
I think Scarlett Johansson did a very good job in Avengers, and despite the basic silliness of having a couple of normal-strength humans in a group which includes Thor and The Hulk, they managed to have her pull her weight and then some. A lot some.
*Spiderman is a big exception, Sony owns those rights, which was why a new trilogy was started so soon; to keep the rights. Too bad, for while I haven't seen it, from what I've read, it didn't seem to be hugely inspired.