Let's start from the beginning. The sequence that plays under the opening credits was a small masterpiece. It was an incredibly effective way to present much of the history that led to the events that played out over the course of the movie. But this sequence is also a source of most of my frustration with the movie, because it demonstrates that Snyder (or the screenwriters) were capable of remaining faithful to the spirit of the original without slavishly duplicating it. If Snyder had taken this kind of approach to the rest of the movie, I think it would have been a much, much better movie. Unfortunately, Snyder decided that he needed to be as faithful to the original as possible, which is not an approach that makes for good film-making. It's the cinematic equivalent of the "living pictures" staged each year at the Laguna Beach Pageant of the Masters. Yes, it's neat and interesting and impressive that real people are able to use elaborate costumes and makeup and clever staging to exactly reproduce works of art, but ultimately you're left asking what the point of it is.
There are two sequences in particular during which I felt Snyder's obsessive fidelity to the original worked to the detriment of the film. The first of these was when Dr. Manhattan first goes to Mars and recounts his history. There are some nice bits in there -- the recreation of Jon's recreation of himself is impressive -- and I think there is some material in that sequence that was important to show, specifically Jon's perception of time and the particulars of the accident involving the intrinsic field
The second point at which I found excessive faithfulness to the original text drawing me out of the movie was toward the end when we saw Veidt with his genetically modified pet tiger. In the comic, Bubastis (a name that I don't remember being used in the film) served as an example of the kind of genetic engineering that Veidt used to create the giant psychic alien squid monster (ah, comics) that was the centerpiece of his grand plan to bring about world peace. Snyder opted not to use the giant psychic alien squid monster in the movie ... so what purpose, then, does Bubastis play? The only answer is that Snyder wanted to recreate the single (admittedly very cool) panel from the comic in which Dr. Manhattan and Bubastis are taken apart by Veidt's intrinsic field
Which, come to think of it, is another example of excessive fidelity to the text getting in the way of the movie. Goode was no doubt cast because of his physical resemblance to the comics character, but unfortunately Goode just wasn't very good in the role. Someone who looked less like Ozymandias-as-drawn-by-Dave-Gibbons but was a better actor would have been a far better choice. I was not hugely impressed by Malin Akerman either, but she wasn't too bad.
Moving on to other matters, the level of violence bothered me. That the movie would seem more violent than the comic was inevitable -- seeing one person hit another person necessarily has more impact (no pun intended) than a drawing of the same -- but Snyder took it to a ridiculous extreme. OK, maybe we needed to see Dr. Manhattan disintegrating that one guy, but I'm not at all convinced we needed to see the literal blood and guts spread all over the ceiling. Likewise, he probably had to show young Walter Kovacs biting that kid's face, but I don't think we needed to see the spray of blood in slow motion. And how about the very first scene, where Veidt smashes Blake's head through a marble countertop and doesn't, you know, cave in his skull. (I guess you could argue that in the movie, Blake may have had superhuman powers; in that same scene, we saw him shatter a stone fireplace with his fist.)
One last, relatively minor complaint: I didn't like Ozymandias and Nite-Owl's costumes. They looked too modern to have been created in the late 60s or early 70s. I know, nitpicky, but I found it distracting.
Having said all that, I still can't say I disliked the film. There were after all many things I liked very much, like the aforementioned credit sequence. And I liked the overall look of the film. You can debate the degree to which Snyder should have tried to recreate the look of the comic on-screen, but having made his choice, he deserves credit for doing a spectacularly good job of it. For some reason, I was particularly impressed by the "Owl-Cave," but it was all good. I thought the performances were, for the most part, strong. Jackie Earle Haley was especially good, especially in light of how much time he spent under his mask. Veidt's new master plan was a pretty good substitute for the giant psychic alien squid monster, and the naming of the plan "Project S.Q.U.I.D." was inspired. Nevertheless, I still have to view it as a missed opportunity.