That was the only sore spot for me, it really should have been the giant psionic alien squid!
Otherwise I really loved the movie and how loyal it was to the rest of the story.
I was fine with losing the squid, and I was overall happy with the translation to cinema, but even though I enjoyed the movie overall I did have two sore spots, the first kinda forgivable and the second less so.
The more forgivable one was the movie's too-literal translation of book four (Dr. Manhattan's soliloquy). The print version of book four is a very experimental book in an experimental series where they try to convey Manhattan's non-linear sense of time by taking normal comic book conventions and twisting them in various ways, in particular by weaving the print narration and the visual artwork in a way that constantly keeps the reader slightly off balance: mentally leading the reader to one place while depicting something else. The many definitions of the word "hands" for example, being played with. The movie can't directly translate that, but it doesn't replace it with anything else. There's no cinematic version of this attempt. And its not like its impossible: Christopher Nolan is famous for playing with the cinematic medium to convey feelings central to the story: Memento being the most obvious example of him playing with how the viewer sees the story to convey a sense of what the protagonist's life is like. Basically, Watchmen the movie loses a moment of printed genius and doesn't even try to replace it with cinematic genius. Disappointing, but forgivable.
The less forgivable one is one I think most people don't care about, but I do. Its a little thing, but its extremely critical to the story. In the books, after the climax when Veidt reveals his plan to the others and they realize they have to keep his secret (except for Rorschach, of course), they don't leave as they do in the movie. They just sort of surrender. The book has a denouement where Manhattan confronts Veidt and tells him he's leaving. Veidt, in his one moment of vulnerability, asks Manhattan "I did the right thing, didn't I? It all worked out in the end." And Manhattan tells him "In the end? Nothing ends, Adrian. Nothing ever ends." And then he vanishes, and its there that Veidt has that hollow look he has in the movie. In the movie, the line is just a throwaway that is delivered by Laurie to Dreiberg where he is the one that says it all worked out in the end and Laurie says words to the effect of "I think Jon would say nothing ever ends."
The scene that is dropped is critical, because its the whole point of the story, or at least Veidt's story. The movie makes it seem like the whole point is that Veidt's plan will be undone by Rorschach's journal, but that misses the actual point of the story in the book. The point in the book was that it doesn't *matter* if Rorschach's journal undoes Veidt's plan. Its that for all of Veidt's intelligence, he doesn't realize that nothing is forever. He is as his namesake Ozymandias trying to build a foundation that will last forever, when nothing does. Veidt is smug and arrogant to everyone else around him, but at the end of the story he shows just the slightest bit of doubt, and he seeks absolution from the one being in the universe he thinks can give it to him: Manhattan. Manhattan's rebuke stings, because in effect Veidt has been judged by the closest thing to God he believes in. Perhaps that's when he fully appreciates that there will never be a time when he or anyone else can look back and judge whether the scales worked out. There is no ultimate high horse upon which to judge morally questionable acts like Veidt's.
Veidt believed his intelligence allowed him a superior perspective to everyone else: that he could kill millions because he had the intelligence to know he was saving billions. That's how he lives with himself committing mass murder. But if there's no way to be sure how many people will ultimately die over the centuries because of his act, and how many will be saved, if its all ultimately dust in the desert, then there's really no intellectual justification at all.
Without this scene, the hammer doesn't truly fall on Veidt, and Dreiberg's words to him as they leave don't - can't - replace Manhattan's words, because Adrian doesn't consider Dreiberg to be morally superior. The movie ends with Adrian believing himself to be a martyr. In the books, he ends as a man whose faith has been shattered.