I'll be generous and give it a C+. The historical impersonations really were as problematic as reviewers made them out to be, especially Nixon. Anthony Hopkins couldn't have made the character believable in that make-up, and the actor portraying Nixon certainly wasn't Anthony Hopkins. For a movie that cost as much as this, this version of Nixon should never had made it on screen.
'Hallelujah' over the Nite Owl II and Silk Spectre II having sex was embarrassingly stupid--and they managed to keep the scene from being sexy, too. And don't get me started over the flame-thrower symbolism. Don't care if it's in the book. Lacking in any form of subtlety--much like most of the musical choices in this film. 'Ride of the Valkyries" over the Vietnam sequences? Really? Francis Ford Coppola must have died just so he could roll over in his grave after seeing these scenes.
Ozymandias was obviously the villain from the get-go--Snyder made no effort to hide this fact and the actor's weak performance didn't help.
I wasn't able to finish the book before school started up again (I only read the first three chapters), but just from those I can see that the film misses out on a lot of details that would keep the world afloat. How the hover ship exists in 1985, for example? How Rorschach's mask works? Nite Owl's laser gun that is briefly used in the finale? Nite Owl's vision goggles? And how did Nixon serve more than two terms, anyway? The paranoid world starts to fall apart without Moore's attention to detail (although I'll readily concede that the ending has been improved, from what I've read).
I will laud the performances behind Dr. Manhattan (the CGI was pretty convincing, too), Rorschach, the Comedian, and both versions of the Nite Owl.
I'll say one thing, though. I'm sure another director wouldn't have made a better film. I shudder to think of a Hollywood-ized version of Watchmen.