While seemingly a fairly faithful adaptation to Alan Moore and Dave Gibbons' influential graphic novel, Watchmen is a very troubled film, both aesthetically and thematically.
Watchmen's major problems lie in Zack Snyder's direction. While the visuals are absolutely stunning, Snyder's inability to get his actors to do more than basic emoting and rudderless direction that doesn't go beyond, "wouldn't this look cool!?" leaves people who are looking for a proper film adaptation unfulfilled.
One of the major issues with the film lies in the interpretation of Moore's script and artist Gibbons' visuals. While the film is visually quite faithful to the book, most of the scenes seem to be interpreted quite shallowly. It seems that Snyder focused in on the fetishization of superherodom and the sexual nature of power as it relates to vigilantism, an easy choice to make, but just one of the many thematic elements in the comic proper.
These kinds of shallow interpretations end up causing another major problem for the film: glossy fight scenes and character moments that are contradictory to the grim and gritty nature of the comic book itself.
The fight scenes in Watchmen are what many people are there to see. They don't disappoint if you are looking for over-the-top gorefests meant to titillate the teenage boys in the audience. Nite Owl and Silk Spectre casually kill people in street fights, far too easily for out-of-practice heroes who haven't seen combat in a decade. Dr. Manhattan explodes people in a completely bloody and visceral way, leaving blood spatters among horrified onlookers.
These kinds of scenes reduce the impact that is so integral to the overall arching narrative. Rorschach is a maddened killer who sees the world in black and white-- the fact that he has killed two people is why he is so feared in the world Moore crafted. But when you see the two most identifiable characters casually stabbing dudes in the neck, it makes both Rorschach look less fearsome and Nite Owl and Silk Spectre to be, at best, much weaker as moral forces in Watchmen's famous ending.
Snyder's macho, double-barreled style also leaves one of the most important scenes in Watchmen to be very suspect in its gendering. During the rape scene between Sally Jupiter and the Comedian-- it's not spoilers if you read the book-- Snyder directs it in such a way that Jupiter wants it and secretly likes it as the Comedian lays a beat down on her for daring to reject him. Flashing forward to the film's present, Jupiter seems almost wistful about it-- a strange interpretation that presents the characters' actions in a whole new, near-misogynist light.
Ultimately, Watchmen is a poor adaptation, which shouldn't be a surprise. Snyder is not known for his ability to work with actors. The Watchmen film is for teenaged boys now and is not the critically-lauded work that helped bring comics into the adult contemporary literature world.