The slavish adaption to the comic made for a long and slow movie. For anyone that's read the comic (and everyone should), this left little room for suprise, as each scene was a direct portrayal of the panels in the original comic. As a result, the film became a less powerful, almost humourous parody of the comic, retaining a small fraction of it's poetic impact. Ironically, had Snyder been more creative in his interpretation, he would have come under more fire for changing "the greatest graphic novel of all time," but there you go. Maybe Alan Moore was right when he said it was unfilmable.
Snyder's attempt to adhere as closely as possible to the actual work, at some points becomes impossible as well. Had they not been such crucial aspects of the original storyline, it may not have mattered so much, but they were. The comic in a comic "Tales of the Black Freighter" and Doctor Manhattan's simultaneous view of time depicted as alternating panels between the present and the past, to name two examples, are elements of the storyline that exploit the comics medium itself, and produce results that are unique to it. This is one of the traits that make Moore a writer who can can transcend the juvenile reputation of the comic book format.
Another problem was what was left out. One of the most sophisticated parts of the comic is near the end where Moore portrays Ozymandias, "The Smartest Man in the World," in front of a wall of TVs discussing the merits of Burroughs' cut-up method and using it to decide what investments to make for his large fortune. Fittingly, this groundbreaking aspect of the comic -- tying the medium to a significant 20th century art movement -- was left out of the movie. Not to say I'm not relieved. I could just imagine all the comic geeks starting to employ the cut-up method without knowing the historical significance of the technique, which can be traced back to the surrealists and Lautreamont. The movie would have been richer for including this scene, but, again, I'm glad it didn't (Beware the DVD...)
Finally, the music was inadequate. Bob Dylan was used three times. "All Along the watchtower" made sense, as it was quoted in the comic, but "The Times they are a Changin'" in the opening scene and the "Desolation Row" cover that play at the end of the movie displayed a total lack of imagination, in my view (as well as a misinterpretation of the vibe of Moore's work). Another great part in the comic that was left out of the movie is when Ozymandias is discussing the new Dub music coming out of Jamaica (not bad for a comic that came out in 1986). Why wouldn't Snyder include some dub in the movie, perhaps when Ozymandias is introduced? No doubt a great opportunity was lost. But frankly, looking at Snyder, one gets the sense that he's be hard pressed to say what dub music is. Again, I'm kind of glad it never showed up.
Having said all this, I don't think the movie was horrible, it just wasn't that great. The Dark Knight was a much better movie. I suspect people that haven't read The Watchmen will enjoy the movie more, though, as the original storyline is so exotic, even a distant echo of it is still an echo of a powerful poetry.