I'm nervous about any logic that compares adapting Watchmen to adapting 300. 300 already was laid out in a VERY cinematic style (someone has said it before, the book itself is already in widescreen) while Watchmen is very very limited in its storytelling style. The narrative is stellar, but let's face it, almost every damn page is a 3x3 rectangular layout of panels. Not very groundbreaking you know? So here's the way I see it: Snyder just had to recreate the look of 300 because the narrative was not really a big deal, and due to the cinematic layout of the book a lot of the work was already done for him. In Watchmen, the look is more subtle and the action is not really the main focus, so in order to translate it to film he's going to need to bring a lot of energy and movement to some kind of stodgy action scenes (I know somebody's bound to argue with me on this one, but really, just compare the 9 step action scenes present in WM to the splash pages in 300 that scream MAKE ME INTO A MOVIE RARRR).
Watchmen's strength and staying power is in its narrative, which in my mind should be easy to capture. I mean, I know everyone else would rather see a miniseries, but I really think there's a central essential story in Watchmen that could be pared down and lifted into the movie, it's not really an ephemeral storyline guys.
Anyway, my point is that Snyder has more work cut out for him because he's going to have to balance the impulse to pump this full of crazy action (which it needs, in moderation) and following the grand story line(s) Moore has given him. Also, I am going to see it just for the squid monster. If they leave that out, I'm burning down Snyder's house.