As far as the story itself goes, the self-conscious attempt of the director to remain as faithful to the source material as possible may have hindered him a bit. Snyder manages to hit most of the biggest plot elements, though even at just under three hours, it seems a bit rushed. The flashback scenes are handled well, and Snyder brilliantly weaves in the songs that have cameos in the graphic novel. But the multi-layered, multi-faceted tale that those familiar with the source material know and love just isn't there. Much of this has to do with what was left out (though understandably so with time constraints), including the highly important sub-story, "Tales of the Black Freighter."
Instead, what the audience gets is a stylized, effects-driven comic-book movie that will leave most wondering what they just sat through for three hours, and why they should have cared in the first place.
Is Watchmen unfilmable? Though Snyder put up a valiant attempt and got many things "right," the film lacks the depth and profundity of the revolutionary graphic novel. So while there may not be a definitive "yes" to that question, it certainly appears that way at the moment.