Iron Man 2 picks up not too long after Iron Man: Stark has revealed his identity as the man behind the suit, and has single-handedly stabilised the Middle East (don't ask). Unsurprisingly, Congress and the Pentagon don't really like a private individual operating effectively as a sovereign power, and are demanding that he hand over the suit technology; Stark is refusing. Meanwhile, over in Russia, Ivan Vanko, the now-grown son of a man supposedly wronged by Stark's father, has got his hands on the original blueprints for the arc reactor technology behind both the Iron Man suit and Stark's heart implant, and is building his own version, with big whippy cutty things instead of guns. And then there's Justin Hammer, CEO of another arms technology company, trying to squash Stark in congress and produce his own rip-off for sale to the government.
So when Vanko tries (and fails) to kill Stark and is captured, you can pretty much write the next two acts yourself, which at least one of the reasons why this film isn't as good as the first.
It gets some things right: there's a party sequence that's utterly cringeworthy for all the right reasons, showing Stark as a largely-out-of-control alcoholic (though Rhodey's response makes no sense), and given that carrying a small nuclear reactor in your chest isn't exactly good for prolonged health, there are both self-destructive touches and thoughts of legacy.
The characters/acting are variable. Sam Rockwell does a marvellous job as Hammer, coming across as slimy, insecure and mean. I'm wondering if he's supposed to be Bill Gates to Stark's Steve Jobs, but I kept thinking Larry Ellison.
Don Cheadle is okay as Rhodey/War Machine. Scarlett Johansson is wasted as the Black Widow: she looks utterly stunning, in every shot (particularly in that catsuit), but she's given almost nothing to do except look stern and aloof. When she finally gets to the inevitable action sequence, there's some weird lighting/camera effects that are probably there to cover up speed issues, and overall I wasn't as impressed as I was with, say, the prison or alley sequences in Watchmen.
Mickey Rourke is great as Venko, though it's probably best not to question why one guy is a top-level physicist and a brilliant progammer and a brilliant mechanical engineer and such a badass, too.
Gwyneth Paltrow is still the heart of the film, as Pepper Potts. Unlike Johansson, she gets a whole range of emotions to portray, and she's fantastic throughout.
Unsurprisingly, though, this is Downey Jr's film. Strangely, playing a self-destructive narcissistic playboy with an interest in martial arts who's managed to grow up a bit doesn't seem to be a stretch. The throwaway delivery is great, particularly in the workshop scenes (which were the best parts of the first film, too).
There are nice nods to the comics fans: the scenes with Samuel L. Jackson as Nick Fury, the teaser scene at the very end of the film - and I particularly loved the glimpse of Captain America's shield, both for its context and for its use. Brilliant.
It's just a pity that that most of the second half of the film is a Transformers slugfest.