The first scenes  show mayhem all over New York, set off by Loki, who survived the events of Thor. Then, we see Nick Fury gather his team together: Iron Man, Black Widow, Captain America and Thor.  Robert Downey Jr. as Tony Stark/Iron Man gets all the good lines, especially in this exchange with Captain America, played by Chris Evans:

Iron Man: No offense, but I don’t play well with others

Captain America: Big man in a suit of armor. Take that away, what are you?

Iron Man: Genius billionaire playboy philanthropist

This is followed by Thor laughing. He had a recent lesson about being humble, and he’s certain to pass that along to Stark.

It’s followed by a lot of action, and Stark giving the mission statement: "It we can’t protect the world, you can be damn sure we’ll avenge it"

The trailer ends with Tony meeting Bruce Banner, commenting on his work, and his ability to turn green and cause a lot of destruction when angry.
 

Bruce is flattered.

While the trailer does suggest the movie might be Iron Man 2.5, future trailers will likely feature everyone equally including Hawkeye, and other characters. For example, we saw Agent Coulter but not Cobie Smulders’  character. Joss Whedon has said he will make it an ensemble movie about a group of super-heroes who shouldn’t work together, but learn how to do just that. It may explain why the first trailer has more Iron Man than anyone.

Check out the trailer on YouTube.