I just finished the graphic novel last night. Besides the character Rorschach, I wasn't familiar with this book at all until talk of an upcoming movie was the cover story on a recent Entertainment Weekly.
The storyline, especially at the time it was written, was revolutionary. Superheroes ("masked adventurers," as they're called in the book), have been outlawed in a 1977 law after a police strike. Their only legal options are retirement or they can work for the government. Depending on how familiar one is with comic books, one may be able to spot certain archetypes in the main characters. There's the gadget-dependent Batman type (Nite Owl), the super beyond-human type (Dr. Manhattan), the sociopathic big-gun Punisher type (The Comedian), and so on.
Each of these characters are more than types, of course. In fact, a big chunk of the story is exploring how they really feel about "adventuring." Nite Owl, for instance, felt silly about dressing up in his outfit. Dr. Manhattan is actually quite bored with humanity due to his powers. The Comedian does his job more for the chance to inflict damage and pain than to help others. Rorschach has an unflinching commitment to punishing evil...his own backstory may be the most involved and complicated.
These types of stories turned the comic book genre on its head because of how human it makes its characters. As it turns out, none of these people really do what they do for truth, justice, and the American way...or at least not in the boy scout manner that Superman does. They aren't so noble and clean-cut. Their motives aren't so pure. Their actions aren't always so heroic. Watchmen was considered revolutionary because of all of this - it deconstructed the genre and explored a much darker, more conflicted side of "masked adventuring."
I first saw the trailer for the movie before The Dark Knight. That was before I read the book. Watching it again, it actually makes more sense. I could even remember back to when each scene happens, so they're trying to stay pretty close to the story. I doubt that hardcore fans would have it any other way.