Let's start with the obvious. The movie is based on a much-beloved graphic novel, a graphic novel that's regularly pointed to as one of the most important graphic novels ever written. There's just no way a movie, any movie, can live up to that.
And it's a graphic novel that's very much wedded to the graphic storytelling format. Not only are its characters and plots inextricably wedded to comics culture, its every panel draws attention to itself as a graphic novel. The promo materials for the movie have gone to great lengths to demonstrate how faithful the look of the movie is to the look of the comic -- but no matter how closely you try to match shot to panel, it's still not the same thing. That famous opening page -- it evokes the idea of a slow pulling-back shot, but it's NOT a slow pulling-back shot. And the slow pulling-back shot they'll probably use in the movie just isn't going to have the same effect.
Similarly, the story is, in a lot of ways, about comics. Comics are embedded into it. There's no way that can be captured on film -- nor should it be.
Let's not forget that Terry Gilliam pronounced this book "unfilmable". And he should know.
Fundamentally, I think the problem is that, based on the evidence of the promo materials (not to mention Snyder's previous work), what we're going to get in this movie is an attempt to slavishly re-create a comic book on the screen. Only with more explosions (there are probably insufficient explosions in the original book, mass carnage notwithstanding). And there's just no way that can work, not with this book.
It's certainly going to be pretty. But when the best I can think of to say is "it's pretty," that's... not really a positive review.
Also, I'm really uneasy about the rumoured addition of an explicit sex scene. WTF is that supposed to accomplish? (see above, re: explosions, I guess)
The only way a movie version of Watchmen could possibly work, in my opinion, would be to re-imagine it as a commentary on movie superheroes -- use the advantages, disadvantages, and conventions of the movie genre, and draw attention to the filmed nature of the work in the same way that the book drew attention to the conventions of the graphic novel. That could work -- maybe. At the very least, it would be ambitious and interesting. And an interesting failure would be worth seeing.
Watchmen is going to make huge amounts of money, there's no question. And I'll be going to see it opening weekend, because how can I not? (Yes, that's why it's going to make huge amounts of money. I know, I know.) But it's not going to be good. There's just no way it can be.
Besides, if I convince myself it's going to suck, then even a mediocre work will be a pleasant surprise...