This movie was a great blend of character development and character action and exposition. It's a fine line that action movies walk when they decide to fully create characters, rather than stick with a flat, one-dimensional character. But Iron Man 2 perfectly navigates that balance. You get those rare, heart-breaking character moments, AND lots of explosions.
Robert Downey Jr reprises the role well - he adds a little more onto the character. Tony Stark develops in a logical way without becoming boring. Pepper Potts remains as adorable as ever (with a few hints of badassery every so often). I was surprised, but I genuinely enjoyed ScarJo in this as well. She's great as a superhero character, and it was amazingly refreshing to have a female character in a hero film that didn't become a love interest. Mickey Roarke definitely wins the "Cool Guys Don't Look At Explosions" award.
Personally, I loved all of the references to the upcoming Avengers plot-lines. They were woven in subtly enough as not to be obvious or obnoxious but they also weren't too obscure.