OK, so now I've read the complete graphic novel of Watchmen, and I'll try to do the fanboy comparison of the book and the movie.
Overall, the movie is a pretty fanboy-ish production. The dialog, a lot of the visual blocking is taken straight from the panels of the novel. What's changed is the double layer of subtext in the newstand/Black Freighter plot, the chapter breaking texts, and the final plot device. As for the first two, that's the price of going from one medium to another. The final plot device (FPD) is actually an improvement. The material in the book supporting the FPD is weak, tenuous, and undercuts some of the suspension of disbelief. The movie's FPD is a tighter construction that helps motivate the self-exile of Jon Osterman, which is still the major plot problem of the whole story.
Watchmen is a wonderful comic book that is in love with the American comic book tradition and is speaking to it, playing with its conventions, and retelling its history. There is no way a movie can do all of that and still be a commercial success. However, the deep ethical question still shines through. Because they operate outside the law, are vigilantes in danger of becoming outlaws? Do they, in the end, become outlaws necessarily?
Ozymandias subscribes to the "shock and awe" theory that led to the bombing of Hiroshima and Nagasaki. To this, Watchmen adds the Big Lie and Orwellian Memory Hole. How many of us would choose that combination over the unflinching stance of Rorshach?