Balance.
Balance is the reason this jam packed Superhero sequel succeeds where others have failed.
By and large a sequel's job is to outdo the original in anyway it can. The easiest and most obvious way to do this often seems to be the "more is more" approach, whereby studios throw as much as they can at the screen often not realising what made the previous film so successful. Some times it tries to cram numerous new characters in, sometimes it's just action overload, but in most cases this strategy backfires. When you can't quite manage the characters you've added into a coherent narrative you get Spider-Man 3, if you eschew any notion of plot in favour of upping the 'splosions and sparks you get Transformers: Revenge of the Fallen. But if you can find the balance, that sweet spot in the middle, that beautiful alchemic reaction, you get Iron Man 2.
Six months after revealing to the world that he is Iron Man, Tony Stark's ego has inflated beyond all proportion. He has declared himself a "nuclear deterrent" and the world's protector having "privatised world peace". Naturally the U.S. government is unimpressed and demand that he turn the weapon over to the American people lest it fall into the wrong hands. Stark's response is that his suit is so advanced that it would take at least twenty years for the world to catch up, "America is safe". What Stark isn't telling anyone is that the Iron Man suit and the technology that is keeping him alive may also be killing him, and what Stark doesn't know is that he may be overestimating how far behind his competitors are.
