So, how do you follow The Avengers, when you only get to do so using 1/6 of the team?
The answer is very simple in theory, though harder to execute in principle: just get out there and make a good movie. And Iron Man 3's not just a good movie; time will tell, but right now I suspect it will eventually be considered the best Iron Man movie. Not because it's the darkest - though it is; unless you count every offscreen World War II death in Captain America, I believe this has the highest body count of any Marvel Studios film, and most are by gunshot. No, it's the best because they seem to have learned from giving Joss Whedon Avengers, and possibly from seeing Sam Mendes put his stamp on James Bond as well - getting a distinctive director and letting him do his thing is a plus, not a minus.
I had some concerns that Shane Black was going to be on a tight leash on this one - at the press day, no one-on-one interviews with him were permitted, just two-on-ones with Marvel's Kevin Feige in the room, which is an unusual arrangement for director interviews. But aside from Disney apparently putting the kibosh on any scenes of Tony Stark (Robert Downey Jr., like you didn't know) being drunk, it doesn't seem like too much creativity was stifled: this feels like a Shane Black movie, albeit obviously a PG-13 one. It also feels like an Iron Man movie, complete in itself and a completion of the character's arc; even the now-obligatory post-credits scene is a throwaway joke rather than a blatant lead-in to the next episode. The movie's already huge internationally, and Marvel will want Downey back, but if they had to, they could leave the character here without leaving anybody too dissatisfied (though a Bond-like tag at the end of the credits simply says, "Tony Stark will return). After all the blatant sequel set-up considerations that killed part 2 for a lot of people (I still liked it, but it does not work as a standalone), this should come as a pleasant surprise.
Black, best known for writing humor-laced action flicks like Lethal Weapon, and directing Kiss Kiss Bang Bang, has a smartass streak that's perfect for writing Downey/Stark dialogue, and an affinity for depressed heroes with demons who can nonetheless be both funny and kickass when they need to be. Here, he gradually strips away every advantage Tony Stark has - the mansion, Jarvis, the suit, the devotion of Pepper (Gwyneth Paltrow) and even the self-confidence: he suffers from panic attacks triggered every time adoring fanboy kids come up to him and ask about the Avengers. Yes, flying a suicide mission into space with a nuclear weapon, then miraculously surviving and crashing to the ground would give a real-life hero PTSD, but that's not something you normally see action heroes onscreen worrying about. So it's a nice touch, and maybe even a convenient teachable moment for the young children of our newest military veterans.
So while Tony is losing all his tools and having to survive on brains and some surprisingly good fighting skills we never really knew he had (and how did he get those, anyway - private ninjitsu lessons at SHIELD HQ?), a terrorist called the Mandarin (Ben Kingsley) emerges, periodically hacking into live TV broadcasts to claim responsibility for random bombings. With iconography that's a hodge-podge of various real-life anti-American villains from Castro to Bin Laden, and an unclear accent that just sounds like fake-American (he is not a "turrrrerist," he says, he is a "teeachurrrr"), this is quite a different Mandarin than the one in the comics, which may bother purists. But once his arc is fully played out, it should satisfy fans of superhero movies, who have never seen a comic-book arch-foe quite like this. There are very, VERY subtle hints that he may be connected to the terrorists who capture Tony in the first film (Yinsen makes a cameo in an early scene set in 1999), but no explicit references are made to that. To say more would be spoiling. [If you WANT to be spoiled, Wikipedia has the complete plot synopsis. I suggest not going there.]
Don't you mean ARE no such THINGS, Sir Ben?
Aside from Mandarin, the movie is basically the Extremis arc from the comics, involving a regenerative nano-whatsit that can help people regrow severed limbs and give them the ability to spew fire and melt things. On the downside, like the blipverts in Max Headroom, it occasionally causes unlucky recipients to simply blow up. Meanwhile, War Machine (Don Cheadle) has randomly been given a patriotic paintjob and renamed Iron Patriot, I guess because having a superhero around named Captain America just wasn't sufficient. Further proving this is the Marvel universe and not ours, William Sadler is the president, with Miguel Ferrer as his veep.