Movie makers, and to an extent, comic writers as well tend to like their characters angst-filled. I think this is because super heroes are sort of an adolescent power-fantasy. Therefore, to make them "grown up", the movie directors tend to play up the tragedy -- which, also, is endemic to super heroes. Examples of this are Spider-Man 3, the original Hulk, and the forced drama in the Fantastic Four movies.
I knew that Iron Man had a Tony as an "alcoholic story line" that the movies were going to touch on, and I feared that the second time, the crew would destroy the fundamental sense of joy that the first movie captured. I didn't want to see Stark systematically alienate everyone and destroy his relationships. Fortunately, this isn't where they went.
To be certain, they do give Stark a self-destructive story-arc. He is now ousted as Iron Man and has to contend with the US Government trying to get the suit away from him. He has a rival businessman, Justin Hammer (Sam Rockwell), who would love to see him fail. He has a mysterious and zealous Russian enemy played by Mickey Rourke. This role would be show-stealing if not for the stellar charisma of Downey. Oh, and he is dying; his power-plant is slowly killing him; and if he can't find a solution for it, he will die soon – but no one knows.
His best friend (Don Cheadle) as War Machine is trying to keep him under control. His mother-figure/potential love interest, Pepper Potts (Gwyneth Paltrow), needs him to take care of his company and is trying to do it herself when he is preoccupied with having fun. Director Jon Favreau plays his driver/assistant, Happy Hogan, who is sort of a hapless observer. Although to be fair, this is probably the best director-cameo I have ever seen. Tony has a new legal advisor Natalie Rushman (Scarlett Johansson) who may be more than she seems...and he has the enigmatic Nick Fury poking around (Samuel L. Jackson).
If it sounds like there is a lot going on here, there is. A whole lot, and as a result, the film doesn't ever really slow down. The good news is that it is built around a series of absolutely crackling action scenes, AC/DC music, and the fact that Downey, with or without the suit, is watchable even when he is buying strawberries from a street vendor. I think the best of the lot was the initial Iron Man/War Machine battle where the two friends slug it out in the Stark Mansion when Tony (drunk) refuses to live up to his responsibilities.
Unfortunately, the movie falls short of greatness. In the final battle, the adversaries just aren't all that interesting, the flying combat runs a little too long, and the show-down with the bad-guy, while nicely short, feels a little anti-climactic. This isn't the worst part though -- the (few) technical flaws would be highly ignorable compared to the bulk of the movie which is fun, light, and interesting.
The major problem is that there is some unnecessary, and frankly, inexplicable stupidity around a "reveal" that is placed in the movie for apparently no good reason. I am okay with Stark building a large hadrons collider in his basement, but the clues that led him there do not make any sense; and there is just no reason I can fathom that they resorted to the trick they used. Without spoiling things, I can't say more; but I really wish that the writing staff had decided to put in another few revisions and come up with something that didn't rely on pretty cool special effects to keep us from groaning.
The Tony-figures-it-out sequence does barely work because it looks good and because we are rooting for him. Given the actor's strengths, I think if the crew had come up with something more coherent and less stupid this turning point could have been really powerful. As it is, the major turning point in the film plays out more like a first-draft sketch note that had all the extra work put in special-effects post production.
The total package, however, is excellent. If Iron Man 2 isn't as smart as The Dark Knight, it is a heck of a lot more fun. If it doesn't direct its dramatic arc as well as the last Hulk movie, it also doesn't bore us with melodrama. The creators realized that we need a lot of Downey, and they give it to us. If Downey's actual history is resonant with Stark's self-destructive streak, that just makes it that much stronger.
Iron Man 2 makes it clear that we are going to see an Avengers movie with Downey in it. Jackson has signed a record-making nine-movie deal. I am reasonably hopeful, after seeing this, that it will be good...assuming they can get around Downey being so charismatic – he is probably going to dwarf everyone else on the screen.