Iron Man 2 suffers from a severe case of sequelitis. It simply had no reason to be made, so much of the plot and even more of the style make no sense. In addition, as is the fashion lately, IM2 borrows heavily from the palette of the graphic novel, much to its detriment. It's a fairly flashy and technically well-done movie, but you could skip it entirely and your life would be just fine.
The movie is a basic established hero being tested by a new villain formula. We met Robert Downey Jr's Tony Stark two years ago - he's the same as before. The supporting characters are about the same as well, although Gwyneth Paltrow's role was amped up and I don't remember her looking quite as radiant two years ago.
Then there's Mickey Rourke. I'm sure he's a decent person but his appearance just bugs me. In case you haven't heard the story, he was a pretty-boy actor a generation ago who took time off for a boxing career. Not a good idea for an actor. He needed some reconstructive surgery and it didn't go well. The guy just looks gross and I may make it a rule to not see movies he's in anymore.
As to the sequelitis, here are a few other things that didn't work.
The bad guy is the son of Stark's father's partner from 40 years ago. When the old guy dies, his son takes out 40-year-old blueprints and a soldering iron and creates a weapon that can take down Iron Man. Waitaminute. If the father had all this information and ability, why did he die penniless and leave the revenge up to the son? See, a plot that makes sense only because they need it in order to make a movie. That's sequelitis.
A sequel needs to have twice the chaos of course, so IM2 has two villains. Sam Rockwell plays a Tony Stark-wannabe and chews up the scenery but doesn't add much to the mix. He looks like he's trying to channel a smooth Brad Pitt but he comes off mainly like a petulant fifth grader.
Samuel L Jackson plays some sort of Justice League mayor who is in two scenes. I don't recall him being in the first Iron Man but he drops by and acts like he owns the place. Very odd.
Jackson's right-hand is played by Scarlett Johansson and it's a totally superfluous character, existing simply to provide a couple more action scenes, as she gets to beat up a passel of henchmen and otherwise be an extra with dialog. Scarlett also played Jackson's right-hand in 2008's The Spirit, although they weren't good guys in that movie. She kept her Catwoman suit zipped up in IM2, but her breasts could easily have been given co-starring credit in The Spirit, as they were on display as much as her face. That's an observation, not a complaint
All-in-all, Iron Man 2 delivers what it promises, but as a sequel to a flash-bang movie, the bar isn't set terribly high.