"Watchmen" isn't a popcorn action film; it's a film that asks, 'what are the psychological, philosophical, and historical ramifications the existence of superheros would bring to the world and themselves?' The film has a plot, but it's only concerned about it in context to the main question the film's interested in.
In the reality of "Watchmen", it is 1985, superheros exist, and America has the only hero with actual powers (that are God-like), and because America has this phenomenal weapon, the cold war rages on as this hero's existence has only made the Soviets fight harder and longer. The war is escalating to the point where Armageddon seems eminent. Nixon is still president, because of this heightened state of war, he was allowed to change the law and stay onto his current third or fourth term.
Besides this God-like hero, named Dr. Manhattan, all the other masked avengers are just people in silly costumes, though each with their own set of psychological problems that being a superhero have caused them - Rorschach is disconnected from the world, only being able to see it's ugly side, The Comedian feels he's a parody of superheros and American patriotism, and Nite Owl II has become a closet superhero after Nixon banned superheros (with the exception of Dr. Manhattan and The Comedian, keeping them on as soldiers in the war efforts).
The film has a mystery plot: a masked avenger is murdered. Rorschach plays detective, suspecting that this murder is going to spread to more hero murders, that there's a superhero serial killer. Anyone expecting the plot to be the central focus, however, will be disappointed. The film spends more time elsewhere, and the mystery plot suffers for it as not enough screen time is devoted to it to be as effective as it could be, and a lot of simplifications had to be made to condense the film's running time, that the conspirator is a lot more obvious than in the graphic novel.
But, that shouldn't be held against the film. It's quite a feat this film was ever even made, and the heart and themes are never lost. The effectiveness isn't quite there, but there was no way it could've been for a film of this budget. To make its money back, it had to be no longer than the 2 hours and 43 minutes that it is. Worry not though, there'll be a 3h10m cut and a 3h25m cut on home video this July or August.
Positives aside, the film isn't perfect. Overall, I forgave it all its imperfections for getting its main themes across (especially the ending, which I'll get to later), and for the countless things it gets right (like many clever devices used to condense the source material - like the intro credits), but I'll still mention the negatives, even if I recommend trying to ignore them while watching the film. First and foremost, it's the gore. I feel it's needed, there was gore in the book, it should stay. But the film adds more and lavishes in it. I've heard that the director quotes Polanski in the reasons for gore in his films, that "You have to show violence the way it is. If you don't show it realistically, then that's immoral and harmful. If you don't upset people, then that's obscenity." Well, anyone who's seen the violence in a Zack Snyder film and the violence in a Roman Polanski film can clearly see that Zack's violence isn't realistic - it's over the top (some say in an artistic way, I say it's in a pornographic way). So, while that's my main problem, that's, what, 2 minutes worth of unnecessary material? And the other issues are nitpicks: some of the music choices seemed off, some of the acting wasn't the greatest (Carla Gugino's acting in old makeup didn't always work, nor did some of Malin Akerman's line deliveries), and the fight scenes were even more stilted than the fight scenes in "300", though, if you liked 300's fight scenes, then the flip-side is that, for you, these are just as awesome, but I personally prefer a more balanced fight where even the great fighters receive some hits.
Where the film's length causes the most issue, is that an important bit is cut to tie the end of the second act better to the beginning of the third act. Which is to say that when our remaining heroes discover who the film's conspirator is, they find information that isn't shared with the audience, so they confront the conspirator with accusations that the audience has no idea what our heroes are talking about. Worry not, soon enough things get fully explained. Still, that bit comes as sort of a hiccup, a brief scratch-your-head-did-I-miss-something moment. You didn't, the film did, but whatever. I'm sure it'll be addressed in the director's cut.
And, for those that have read the novel, the ending is different, yet sort of the same. The differences had to be made, as to set up the novel's ending, the film would've needed to include the pirate comic book writer that mysteriously goes missing, and the film just didn't have time to devote to him. So, don't expect to see any tentacles, and giggle at the clever campiness of it. Instead, the new ending has religious connotations to it, and the essence and moral conundrum is still there.