Iron Man 2 is a classic case of "too many good ideas for one movie." From the "making of" documentary and featurettes for the film, it is evident that director Jon Favreau felt rushed to complete the film in order to meet Paramount and Marvel's release date, and the wear on him physically and mentally is apparent. He also mentions how little time they have, how the script isn't quite finished, that costumes aren't done, effects shots and casting come down to the last second. There's a moment where screenwriter Justin Theroux comes in to help feed lines to Sam Rockwell in order to set up the War Machine subplot. Based on the pressure to keep things moving and the suggested lack of connective tissue, I would wonder if that was more the norm than the exception.
Frankly, it's impressive that Iron Man 2 holds together at all. There are at least four competing stories in Iron Man 2 that don't always seem to fit together. Let's take a quick look:
1. Tony Stark's greatest invention is killing him, and his friendships are suffering. The only hope is to reconnect with his deceased father in order to discover a way to stay alive and do good, but he's running out of time to find a stable substitute as his celebrity continues to rise.
2. The Stark family history (including the Expo) hide a secret about what Howard Stark did to get where he got and the people who were stepped on along the way, including the Vanko family. Not only does Tony have to continue to be accountable for the Stark history of warmongering, but he faces a very palpable threat in the guise of a wronged man and a rival weapons dealer.
3. Tony has to deal with the possibility that not only can the Iron Man suit end up in the hands of his enemies, but also his friends. When "Rhodey" takes a Mark IV suit with him and returns it to the military, they immediately set about working with Hammer to create a "War Machine" along with Hammer's drones, and use Stark Expo to unveil their new weapon.
4. Stark's place in the larger Marvel Universe if offset as the film sets up The Avengers by introducing Johansson as Black Widow (I'm assuming you've seen Iron Man 2 by now or at least the trailers for The Avengers so that's not a spoiler) and increasing S.H.I.E.L.D.'s role in the film with Coulson and Fury making extended cameos in the film. There are also direct references to the events of The Incredible Hulk (which appears to happen simultaneously with the end of Iron Man 2) and the arrival of Mjolnir - Thor's hammer - on Earth at the end of the film, a tease that has no direct bearing on Tony Stark.
All of this happens in two hours, and that's not including the first fight with Whiplash that derails a Grand Prix in Monte Carlo, a prison break for Vanko, an action scene for Black Widow (and, for comic comparison, Happy Hogan), and the showdown between Whiplash, his drones, and the Iron Man / War Machine tag team, also during the Stark Expo. The various plotlines are resolved haphazardly, if at all (the Justin Hammer character basically disappears in handcuffs promising revenge) just in time for Tony and Pepper to get together after his new suit saves his life. Black Widow doesn't really make much of an impression at all, as there's not a lot of time for an arc for Johansson in someone else's movie. And hey, there's Agent Coulson in New Mexico! He found Thor's hammer!
There aren't many "villains" in Iron Man 2, at least in the conventional sense that Jeff Bridges' Obadiah Stane was as Iron Monger in the first film. Ivan Vanko is a man who comes from a family wronged by Howard Stark and is justifiably angry with Tony's glorifying being Iron Man. Justin Hammer is a guy who desperately wants Tony Stark to be his friend, or at least consider him as an equal, to no avail, so he acts out in foolish ways to get even. Even Rhodey only steals the suit because Tony is being a drunken mess during his birthday party and using his own suit to blow up bottles and watermelons. The drones are just that - drones. You don't really feel anything for them so it's no big thing to see them wiped out en masse at the end. Even the perceived threat of Stark's mortality (played up repeatedly in the film) is a straw man, because we all know he's not going to die in the second movie.
So how is it that Iron Man 2 works? How is it that, despite the overstuffed narrative and the rushed schedule and the various "needs" being addressed to facilitate more Marvel films, that I still mostly enjoy the film? Well, it is still funny - Robert Downey Jr. brings a sardonic quality to his impending demise, and his conversations with Jarvis (Paul Bettany) continue to make me chuckle. Likewise the relationship between Stark and Coulson, which actually figures prominently into The Avengers (kinda SPOILER), and the scene where a hungover Tony meets Nick Fury at a doughnut shop. Favreau and Theroux poured every ounce of themselves into the film, as did the cast and crew, and despite the fact that Iron Man 2 just shouldn't work it almost makes it.
It doesn't collapse on itself (like Captain America) and doesn't lopsided in one direction (Thor). The Incredible Hulk's connection to the Marvel world is more tenuous - watching the film again I noticed that all mention of Stark Technology and of S.H.I.E.L.D. appear during the opening credits and the Super Soldier Serum is mentioned by name only once. Were it not for Robert Downey, Jr. appearing at the very end of the film to talk to William Hurt's General Ross, there'd be no tie between Iron Man and The Incredible Hulk at all. That's not really on Iron Man 2, which has a "blink and you'll miss it" screen of footage of Hulk's college campus showdown with the military, but one does get the impression that Marvel felt the need to overcompensate now that the stage had been set.
I'm curious to see Shane Black's Iron Man 3, which has the opportunity to largely be free of the need to set up other Marvel heroes or to feature S.H.I.E.L.D. so prominently (that's what The Avengers 2 is for), and to see where they go with Iron Man's rogue's gallery next. Favreau left the series (and The Avengers) citing a disinterest in moving into the "mystical" realm, although watching the extra footage, I wouldn't be shocked if exhaustion didn't factor in as well. One of the last things you see him doing is recording the audio commentary for the film - before Iron Man 2 is released. He admits it feels surreal to be speaking about the finished film without any context of its release, reaction, or with any distance from its production. Iron Man 2 was a race to the finish line, and it just manages to cross before collapsing.