I'm sure by now we've all heard the same discussions, pacing may have been an issue, and of course the ending. But in my opinion, Watchmen was never supposed to actually be about the plot, but the messages that the characters and plot convey. If they do that, than isn't it successful?
Of course, there over-arching theme to the movie is, what is peace worth? Is worldwide peace worth the death of millions, or as Veidt says,"Millions dead for Billions". I suppose there is no right or wrong answer, it depends wholly on you're perspective, Machiavelli or Montesquieu? Either way, this question is undoubtedly addressed, and in my opinion more so in the movie than in the novel; the movie seems to force the idea down your throat while the novel lets it sink in.
The next question is the infamous, "Who watches the Watchmen?". Again, I think that this issue has more of an impact in the film genre. Its one thing to read of a character who kills millions, or burns someone alive (in the movie cleaves him in the head), as opposed to the heroic Nite Owl II who simply wants peace. This idea is more fleshed out when you actually see the contrasts in thought and motivation acted out. I'm not arguing that the novel isn't successful in this, I think it effects the audience in a much deeper sense. However, in the film, there is a more immediate, emotional impact which isn't seen as directly in the novel.
A that my girlfriend brought up that I hadn't noticed, which I've also read in some reviews, is the apparent lack of interest in make up and hair (wigs, etc.). Maybe I'm giving Snyder too much credit here, and I'm letting my snooty Art History background come through, but I believe this move was on purpose. The movie is centered around "the joke"; the falseness of society and that through attempting to be civil we kill ourselves. Thus, if the movie is centered on false details in the world, than wouldn't wigs, makeup, and Nixon's nose looking false subtly enhance the undertone?
Overall, while I did like the novel better, I do think that the movie was successful in the philosophical sense of the novel. Were important scenes cut that could have enhanced the movie? Yes. However this didn't make for a bad movie and I would argue that it needs to be seen at least twice for optimal experience.