This was originally written ages ago but I didn't have time to finish it off then, so here it is now...
Although a lot of what I wanted to say is along the same lines as Mark Kermode, I did have a few thoughts on it of my very own that I've decided to note down here.
First off: I really liked the graphic novel, so I suppose this may have coloured my impressions slightly, but only to the extent that I am aware of the quality of the source material, not in a they-changed-the-ending-how-dare-they way. If it had done something interesting with the story or changed the plot significantly I wouldn't have minded - the important thing is: as a film, is it any good?
Second and final caveat: I went to the film aware that it had been directed by Zak Snyder, he of 300 and the Dawn of the Dead remake. So I already expected it to be stylish but vacuous. Again, in a specatacle, blockbuster type movie that's not necessarily a bad thing, I'm not a prig about pure spectacle (see my unabashed love of the Transporter films for example).
There were all sorts of problems with it. It was boring and overlong. It had that feeling of weightlessness you get with action films - CGI and choreography combine to make suspension of disbelief harder to achieve. It had a lot of people talking in rooms, to the extent that I started to wonder if it wouldn't have made a better Radio Four drama serial, one that could have been played in Woman's Hour for a week.
In the end though my major problem is not that Zak Snyder makes glossy action films. It's that he makes glossy action films when he's been given material that deserves better. Dawn of the Dead, a brilliant satire and comment on consumerism, was remade as a standard zombie-fest. Fine, but without any of the ideas that made Dawn interesting in the first place. The Battle of Thermopylae - a broad historical drama that could've been given the full Rome treatment but instead was about copper-coloured skies and shouting. Watchmen, a brilliant satire on the narrative conventions of comic-book stories and a comment on the psychological and sociological implications of the phenomenon of the superhero, became hours of leaden dialogue, slick action and a rubbish softcore sex scene (although, it was the one occasion in the film where both I and my companion got a genuine laugh, so there is that to be said for it).
Snyder doesn't seem to understand the language of cinema or his source material. He can put a film together, but that isn't enough. When the Watchmen go into action, there they are knocking out the martial arts stylings of a Matrix junkie, for no reason that I can understand. But then, to show they've got all depth and shit, yeah, he gives them huge swathes of uncomfortable dialogue that with good reason the actors speak sounding like Angus Deayton in his HIGNFY days (i.e. autocued up to the eyeballs). Having them talk about determinacy does NOT give characters depth, it just makes them sound like pretentious sixth-formers trying to impress the common room, and does nothing to help you understand them or their kick-ass fighting powers.
I could go on, but it would get into more subjective territory, such as the decision to glorify the violence that the Watchmen indulge in, something that the source material doesn't do and which undermines the key points Moore is making. In the end, it was Snyder's choice to make his film morally bankrupt and it seems unfair to criticise him for not putting Moore's book straight up there on screen. It is another reason not to bother to see it though, because if you want to see a film that makes ultraviolence look way cool, but without also boring the shit out of you, you can always watch Transporter.
I also won't dwell on the sexual politics of it (tied into the Sin City aesthetics amongst much else). They are hugely retrograde from the source material, but you do expect that with this genre, and especially with this director, who just doesn't seem to understand the meaning of his own work very well.
Please don't misunderstand me, these are all reasons to dislike the movie, but for me its biggest failure was that it failed on its own terms well before it offended my personal sensibilities.