(Just a little rant inspired by many things, including io9 recently putting IM2 on their "worst of" sci-fi list when "Predators" made the best") 
Why is
IM2 a good movie? Glad you asked...
The Script/Story
IM2 tells the story of how Tony's self-imposed social isolation becomes too much for his body to handle as the only Iron Man, and how, with the help of some cooler heads, he regains his mojo just in time to prevent a psychotic and an amoral arms dealer from proliferating WMD-like technology and killing the one woman who sees him for his potential inner worth.
Yeah, I'll take that over "rich douchebag with a tank but nothing to say fights a guy planning to infect a city's water supply with hallucinogens with a magic microwave that'll boil the water but not people's bodies" any day.
The Cast
Let's get one thing straight:
IM2 isn't the Keanu-tastic
Matrix sequels or even
X3, where apart from Jackman, you're saddled with Kelsey Grammar and Halle Berry at their most tedious, and need a razor-sharp, first-rate script to stay afloat. No, the pairing of the always awesome RDJ with the awesome-with-RDJ Paltrow inherently elevates this sequel to memorability. And no, that's not to be taken for granted.
Moreover, you get Mickey Rourke as a crazy, tattooed Russian, Sam Rockwell at his kookiest (dropping a James Joyce reference!), Samuel L. Jackson with an eyepatch, and ScarJo in tight leather, to say nothing of Cheadle, that senatordude and the legendary Coulson. I call that an embarassmeant of riches
This movie, like
The Wolfman (but more so), pretty much succeeds on the strength of its cast alone.
A Sense of History
How often does a superhero movie engage with history in any meaninful (let alone interesting) way? To most tentpole pics, "history" either refers to strictly personal family history (an anecdote or two of Aunt May's about Uncle Ben), or total bullshit (an alien robot frozen inside the Hoover Dam). IM2, however, gives us plot points integrally tied to the post-War economic and technological boom, the military-industrial complex, the Cold War and World's Fairs, with hints of Disneyesque Americana and the inherited weakness for alcoholism. Even its outlandish present (North Korea building WMDs) engages with reality rather than ignoring it in favor of some urban decay fantasy (
The Dark Knight and its preposterously powerful mobs) or random/anonymous shiny city (
Superman Returns,
Tron).
This, folks, is that rare gem: a blockbuster with brains.
Avengers Stuff: Tease?
Lots of IM2 complaints have centered upon the Avengers material, calling it superfluous to the movie's story and mere shilling for the upcoming Avengers movie. Both points are crap
First, the notion that the Fury/Black Widow scenes aren't important to the plot. This is the first time since college that Tony has had cause to look up to anyone as any kind of authority figure, and they represent the first group (apart from the Taliban, naturally) that hasn't welcomed him with open arms
in his whole life. When Tony sees how close the Avengers are coming to muscling him out of Stark Industries via the Pepper-Widow business alliance, he's inspired to get his act together and grow up a bit. Fury thus doesn't
just advance the plot, he helps our protagonist mature. Take out the Avengers scenes and you've got a much flimsier story - unlike the Gambit/Blob/Electric Hobbit stuff in
Wolverine, which
was mere plot filler.
With this in mind, as for the notion that those bits are building to the
Avengers movie: so the heck what? What Marvel is doing with this shared cinematic universe of theirs is literally unparalleled in more than a century of film history. It's innovative and exciting, and even if the Avengers movie falls flat on its face, isn't it more admirable to have too much artistic ambition than too little? The vast majority of superhero movies are still designed to inhabit one isolated franchise, and I therefore frankly welcome something different
The Action
It was pretty damn cool. Also, ScarJo in tight black leather instantly invalidates any and all possible conflations of this and
Spider-Man 3.
Not Perfect
Granted,
IM2 wasn't perfect. I would have liked to see Sen. Stearns identified as a Republican, Tony's poisoned veins healed
way too quickly
twice, Happy and Pepper driving against the race traffic was as dumb as the bronto chase in PJ's
Kong (albeit far, far shorter), and the suitcase suit would have worked better as a skeletal frame similar to Vanko's suit. But these are minor quibbles
Conclusion: enough with the whinging.
Iron Man 2 was a damn fine flick.
. I also really enjoyed both sequences with Garry Shandling's Senator Stern. The first sequence more for RDJ, but his line at the end about "how annoying a little prick can be" makes me LOL every time I see it. great delivery and build up.
