My only major problem with the film is that the gore seemed excessive at times. I realize we're talking about a dark story and about Rorschach, who is basically a one-man reaving factory, but there were moments it seemed we were supposed to be getting a kick out of the ultraviolence, and that didn't really seem appropriate. There wasn't anything in totally poor taste, but... maybe it could have been eased off a little.
(I did find it bitterly ironic, though, that the one moment in the story that there should have been goresplosions, we instead get very clean explosions/disintegrations instead. I imagine it fits the plot change, but it struck me as sort of a weird directorial choice.)
As for the plot change: does it make me a bad person that I found it appropriate? It struck me as a reasonable narrative tightening, and arguably the plan makes more sense than the original novel version; after all, there are no real aliens, but there is a real Dr. Manhattan to serve as a deterrent of indefinite duration. Overall, while I was worried the movie would treat Ozymandias poorly, his depiction seems reasonably balanced -- the new suit is atrocious, but at least he's not a goddamn Nazi.
A lot of the Internet discussion of this movie has involved the "Hallelujah" sex scene and Dr. Manhattan's nudity, and to both counts, I find myself wondering what all the fuss was about. The sex scene objections I can basically understand -- it does run a little long (although not as long as I was picturing), and it is a little gratuitously Movie Sexy after a first scene that felt very real and character-appropriate -- but I don't get it as more objectionable or in more dubious taste than, say, any given scene involving Rorschach in prison. As for the Dr. Manhattan junk jokes... seriously, random dudes on the Internet? This is what you were exploding about? Occasional frontal nudity of an idealized male form, generally at a tasteful distance? Way to embody every homophobic Internet-dude stereotype!
... Sorry, but come on, that was nothing.
Closing thoughts: I saw this movie with my dad, who has never read the original Watchmen, and I was worried that some of it might be awkward (particularly after the Internet made me think this movie would contain like fifteen minutes of nonstop blue junk-swingin'), but it ended up being okay. He was surprised by the darkness, though, and also by the fact that Ozymandias didn't die at the end. On the way home, he asked me a question I really had no good answer to: "so, why didn't they kill him? Were they setting up for the sequel?"
Yeah, I had nothing.
I should probably reread the original graphic novel now, honestly. It's been years, and I'd forgotten big chunks of it. So it goes.
... I also may have to buy the soundtrack. Still have no idea what to think of the end-credits cover, but the rest was well-used and very nice.