Director Jon Favreau adeptly balances the story and character development, wisely keeping the action from becoming too overbearing (a particularly laudable feat, especially in this genre which has become quite overwrought in that department). The scenes in which Stark is building the suit are just as entertaining as the ones in which he finally gets to use it. Favreau knows that he has a gem in Downey's performance and uses it to outstanding advantage, allowing the climactic pyrotechnics to have emotional and narrative resonance.
And make no mistake, the key to the success of the film is in the much-touted casting of Robert Downey Jr., whose notoriety works beautifully with the character he is playing. More importantly, he is obviously having a great time playing Tony Stark, who himself is a person determined to have as much fun as humanly possible (a good part of the appeal of the film is that Stark gets to play with the coolest toys). It is infectious, and Downey's wit and screen presence manage to draw the viewer into the story; at no point does one question Stark's considerable intelligence nor his motivations. He can also be something of an asshole at times without losing audience identification.
Gwyneth Paltrow is on hand to provide an interesting alternative to the tired love interest. Her loyal-to-a-fault Pepper Potts is a woman who knows Stark too damn well to let him get close to her, and the film smartly does not overplay this relationship. Jeff Bridges essays a neat turn as Stark Industry's company man Obadiah Stane, and though Terrance Howard's Jim Rhodes isn't given much to do in this film, there is always "Next time, baby."
Matthew Libatique's cinematography is excellent (the print I saw looked much crisper than most Super 35 fare) and J. Michael Riva's production design is suitably slick. Ramin Djawadi's music is unfortunately another example of standard-issue by-the-numbers Media Ventures/Remote Control drek which doesn't hurt the movie but doesn't help it either, but the songs included in the film are always spot-on. The effects are outstanding; interestingly, the look of the suit when Stark initially dons it has an appearance that comes across as overly digital, but this is not a bad effect, it is just that spotlessly clean metal surfaces just look like that. The moment the suit begins interacting with the real world and gets some texture, such concerns disappear.
The film is clearly the first of a franchise, which is perfectly fitting considering the serial nature of the source material, and there are a few hints of what is to come that sound pretty exciting (and this is coming from somebody with next to no background with the comic books), but these bits are not at the expense of the enjoyment of this movie. If anything, they are welcome because this movie is such a wild ride that it leaves you ready for more.
The movie has an tag at the end of the credits that is not to be missed.