I do not mean to imply that this movie was made solely of bad parts. To imply that would be a lie: there were several parts which shone through brilliantly.
The actors for Rorschach, Dr. Manhattan, and the Comedian fit their roles perfectly, pulling them off better than I could have hoped for. The parts for the characters themselves were very well written, too, and what little exploration was done of their pasts and motives was handled very capably and even-handedly.
The Comedian, in particular, was handled masterfully. I don't think he could have been put to film better by any director in thei ndustry. There were tiny touches throughout the movie - like Nite Owl II's mention of how the Comedian once used the flamethrower in the same way as Silk Spectre - which wasn't even in the novel, but showed how Snyder put in a little nod to the Comedian and Spectre's relationship. The man clearly loves the material he was working with, and is a very capable director.
The problem lies in the material itself. The Watchmen is made up of some very good parts, but the limitations of cinema means that it cannot be conveyed as the book was - it can't be helped, but it meant that the adaptation was its own worst enemy.
It's a shame, because Zack Snyder did very good work making something that I think is terrible.