In all honesty, I wasn't expecting that much from this movie. The first one was enjoyable, but suffered greatly from the emphasis on origin story and lack of villain. Though there is still no real plot, this time around both Mickey Rourke and Sam Rockwell step in to play interesting foils for the titular hero. The film is fast paces and visually very appealing. Like in the first movie, many of the best scenes are those not featuring robot smashing action, but those involving Robert Downey Junior acting out as Iron Man. With that said, the action scenes are very well done and exciting.
The film draws on a lot of elements from the larger Marvel Comics universe with Nick Fury, The Black Widow, and S.H.I.E.L.D all making appearances. The villains are also drawn from the comics, though with some significant changes that seem to have been made mostly in response to the casting of Sam Rockwell and Mickey Rourke (not that I mind really). Mostly these elements add some richness to the universe that Iron Man and company inhabit. We get the idea that there are other interesting things, such as the Incredible Hulk rampaging through a city and Captain America's shield being rebuilt, happening that could lead to cool adventures down the road.
However, as cool as as all these easter eggs are, I think ultimately they distract from the main plot too much. There is an entire subplot about a mysterious event in New Mexico that is included seemingly as buildup for the upcoming Thor movie, but is completely orthogonal to the main plot and never adequately resolved. Comic fans might understand the significance of hinting at Thor and Captain America in an Iron Man movie, but its just confusing to everyone else (and I thought it was all handled pretty sloppily in terms of writing).
Criticisms aside, Iron Man 2 is really an enjoyable summer blockbuster. The cast is good enough to overcome most of the weaknesses in the writing and the action scenes are almost shockingly exciting.