The casting is perfect. Ozymandias (Matthew Goode) looks younger than in the book, but it works. Jackie Earle Haley as Walter Kovacs/Rorschach looks and sounds like a scruffy, violent redheaded Clint Eastwood, and he comes up with a perfectly plausible way to go "hurm". (It's just a grunt, basically.) The guy playing Richard Nixon looks a bit cartoony with his latex nose and jowls, but then Nixon was always pretty cartoony looking anyway.
This movie also allows you to witness Lee Iacocca getting gunned down, and the odd sight of Nixon as the good guy!
Stuff they changed:
Dan, not Rorschach, goes to tell Adrian Veidt about the Comedian's death.
When Rorschach describes what he did to the guy who kidnapped and murdered the little girl, instead of setting him (and the building) on fire, he chops into his head with a meat cleaver several times.
Ozymandias's plan involves not a gigantic space squid, but a massive energy blast made to look like the work of Dr. Manhattan. The effect is the same: half of New York is wiped out, yet Dr. Manhattan doesn't seem too upset about it (even though, in the movie, he's been framed for it!)
ETA: Ha! Just noticed an interesting minor change. On the last page, Seymour at the New Frontiersman says, "Robert Redford says he'll be running for President in '88." In the movie, "Robert Redford" was changed to "Ronald Reagan".
About halfway through, I had to go to the bathroom, so I went out into the lobby, went to the bathroom, and then went into a theater that had started showing "Watchmen" an hour later than the one I was in, so I got to see about an hour of it over again. At first I was a bit disappointed because the first audience had been full of fans who were clearly enjoying it, raising a fun geeky vibe, but the second audience was absolutely silent. Also the second screen was a tad splotchy.
But then a scene that had been clumsily cut out in the first theater, perhaps because the film had been damaged, was present in the second theater. At that point I was glad I'd gone to the bathroom! (The scene was the one where Laurie and Dr. Manhattan are making love, and she suddenly realizes there are two Dr. Manhattans in bed with her. The film cut awkwardly from the end of the previous scene to the moment where she's walking out of the bedroom.)
Trailers I saw:
"Knowing", starring Nicolas Cage as a guy who finds an old sheet of paper with a series of numbers that, when decoded, apparently predicts the end of the world.
"Angels & Demons", the sequel to "The Da Vinci Code" in which Tom Hanks goes up against the Illuminati. Caught a few glimpses of ambigrams by John Langdon.
"Up", Pixar's next movie, with more scenes than the teaser. Still don't know where the boy scout was when the house takes off; he's not visible on the porch at that point.
"Observe and Report", starring Seth Rogen as a mall cop, and who I first thought was Britney Spears but is apparently actually Anna Faris.