Most of the negative reviews have been agreeing with Alan Moore's assertion that it's an unfilmable story. Maybe I don't have the deep connection to the source material that some of these people do, but for me if it's an entertaining two and a half hours that maintains most of the tone of the book I'll be ecstatic.
Also, I hope there are explosions. Lots of explosions.
**UPDATED** Read what I thought after "Watchmen" at http://preview.tinyurl.com/b8yj38