I read the comic. And frankly, I´m kinda concerned. It´s not ment to be an action blockbuster. The story in the comics is pretty bleak. It deals with some pretty heavy topics. The creator, Alan Moore, was trying to show that in the real world, heroes wouldn´t be so heroic. I´m afraid they will completely tone it down and make it into an action film.
From the top of my head, the story is basically this. The Watchmen was a group that existed a few years before the story starts. One of them dies and the main character, one of the "heroes" named Rorschach thinks it´s a conspiracy, that someone is trying to kill the Watchmen.
From Wikipedia (http://en.wikipedia.org/wiki/Watchmen):
"Watchmen is set in 1985, in an alternative history United States where costumed adventurers are real and the country is edging closer to a nuclear war with the Soviet Union (the Doomsday Clock is at five minutes to midnight). It tells the story of a group of past and present superheroes and the events surrounding the mysterious murder of one of their own. Watchmen depicts superheroes as real people who must confront ethical and personal issues, who struggle with neuroses and failings, and who—with one notable exception—lack anything recognizable as super powers. Watchmen's deconstruction of the conventional superhero archetype, combined with its innovative adaptation of cinematic techniques and heavy use of symbolism, multi-layered dialogue, and metafiction, has influenced both comics and film."
The phrase "Who watches the Watchmen?" was an inspiration for Moore. Also, the end of the comic is pretty depressive too. I think they will change to make it more acceptable to the mass-market.
One more thing, in the world of Watchmen, nobody has superpowers. Except for Dr. Manhattan (the blue guy in the trailer). And he has such amazing powers (God-like powers, like time-traveling, altering reality) that he kinda slowly loses his humanity.
But, it is a very good graphic novel. Just don´t expect your traditional good vs. evil of DC and Marvel.
P.S. - There is meta-fiction in the grphic novel. There is a story called "Tales of the Black Freighter". It´s meant to simbolize some of the things that happens in the history.
P.S. 2 - Watchmen is considered to be the greatest graphic novel ever. Even people outside of comics thinks that this a great novel.