Not gonna lie, kind of disappointed by Iron Man 3. I was expecting it to be darker and perhaps more vulgar when Shane Black was announced as director, and while that won't bother most people it kind of tarnished the experience for me. What I liked about the first film (and for that matter Avengers, Cap and Thor) is that it was breezy. There is a serious dearth of cheerful/pleasant movies that aren't aimed exclusively/ostensibly at children. Marvel can do what they like with their franchises, but I think that's a niche worth filling. Given the direction they've taken the Iron Man trilogy and what we've seen of Thor and Cap 2, it looks as if they're not interested in continuing with the tone they pioneered in the introductory films.
The dark elements might just exist in the way I perceived certain ideas, but it certainly doesn't feel like Iron Man 1. The film is based on two modern storylines that I personally disliked: Extremis, for how dead and bleak it felt, and (more to the point) Five Nightmares, in which arc reactor tech is reverse engineered and weaponised, turning people into supercharged suicide bombers. It's an idea I found distastefully presented on the page, and no less so here. The concept is sound and it's not thematically alien to the character, but it does alter the atmosphere to a significant degree.
That said, Iron Man 3 is full of really great moments and performances. I was always engaged and it was hilariously funny on a frequent basis, it's just in retrospect there's not a lot to latch onto story-wise. If you've followed the marketing you'd get the impression the movie's about Tony overcoming his PTSD after the Avengers. Watching the movie itself you might develop that impression on your own, but the issue is never resolved, or rather (to talk around a spoiler) it's resolved in the most backwards manner possible in that it becomes irrelevant and will presumably be ignored from here on. It just felt unfocused. Without the PTSD acting as a unifying theme, you find yourself asking what the entire thing was really about.
I'm going to put this in perspective with a convenient list of Marvel films, from most to least enjoyable (in my opinion):
2- Iron Man
4- Captain America
5- Iron Man 3
6- Incredible Hulk
7- Iron Man 2
...Holy shit, there've been 7 of these things now? To be honest my ten year old self wouldn't give a damn about any of these complaints, he'd just be amazed that 7 decent, popular, and still somehow faithful movies have been made about these characters, with more to come. I think what bothered me is that this is the first one that made me worry they won't be as decent going forward.
Edit: I saw one review mention that the heart of Tony's problem in this film is that after his self-sacrifice in the Avengers, the formerly egocentric hero can't stop the self sacrifice, resulting in a new outlet for his self-destructive tendencies and manifesting as obsession, recklessness and anxiety. I find that with this in mind I like the film more and the resolution makes more sense to me, with the caveat that it's still a little too abrupt and extreme a decision for him to make, and one that I still think is out of character.
While I'm editing this, I should add something that I forgot to mention. He says he's building all the suits so he can protect Pepper, the one thing he cares most about. Wouldn't the logical thing to do be building a suit for Pepper? Y'know, the Rescue suit that every nerd really really really wanted to see in the film? I don't know exactly how he envisioned the space suit (that was heavily teased, then never used and barely shown in the film at all) coming into play, but I can't imagine it was built to benefit her. I'm honestly kind of pissed they promoted those suits so heavily and then didn't follow through on the implicit promise there.