Silk Spectre I: Susan Sarandon or Helen MirrenObviously, Snyder decided to cast younger actors in the role, perhaps because it would have been easier to age them in the film than too make older actors look younger in flashbacks. That was a mistake, as it turns out. Of his original cast, I would only have kept Haley as Rorschach, although my two other alternatives would likely have been as good, if not better. Carla Gugino was not bad as Silk Spectre I, but again, she was just too young to play that role. Hoffman is far more in line with the Nite Owl II of the graphic novel than the fit Patrick Wilson. Just about anyone would have been better than Matthew Goode as Ozymandias. Just think: If Pitt had been cast in that role, perhaps Angelina Jolie could have done the micro cameo as Silhouette. And how could Anthony Hopkins not return to play Richard Nixon, after playing the same role in Oliver Stone's 1996 film?
Silk Spectre II: Kate Winslet
Nite Owl I: Clint Eastwood
Nite Owl II: Phillip Seymour Hoffman
The Comedian: Mickey Rourke
Ozymandias: Brad Pitt or Chiwetel Ejiofor
Dr. Manhattan: Viggo Mortensen
Rorschach: Jackie Earle Haley, Sam Rockwell or Gary Oldman
President Richard Nixon: Anthony Hopkins
Moloch the Mystic: Sean Penn or William H. Macy
Speaking of Oliver Stone, say what you will about him, but he would have made a stellar director for this film. Watchmen covers a lot of ground and flashes back many decades to establish the complex back story. With JFK and Nixon, he illustrated that he can cover a lot of history in a short amount of time using a number of cinematic techniques and camera types.
For further reading on this subject, check out the March 2009 review of Watchmen done by now defunct pop culture blog, The League of Melbotis, and then venture back to the summer of 2004 and read The League's prescient thoughts on the perils of adapting Watchmen to the big screen.