I would point out, however, that the story is extremely simple and Peter Jackson had three films and something like 9 hours to tell this tale.
We must keep in mind that Watchmen is not prose and that a picture paints a thousand words. Watchmen would not work in prose any better than in cinema, but if you were to translate/adapt the comic book into a prose telling, being sure to add in completion every detail, you'd likely end up with something about as long as the three LOTR books. They'd be a godawful mess, but I expect it would take that long to cram it all in. Every detail. Every background nuance. All the layers and symbolism. All the material at the end of the chapters. Everything.
I disagree on both counts.
1) With the LOTR books, Tolkien developed a huge, sprawling world with many different species, dozens of languages, and well over a hundred characters. If that's not enough to convince you, consider this: The LOTR series spanned three books. Each book got a three-hour movie. Let me repeat that: EACH LOTR BOOK GOT THREE HOURS OF SCREEN TIME APIECE TO TELL THEIR STORIES. And a whole ton of stuff still
ended up on the cutting room floor.
Does it still seem like an "extremely simple" story? That said...
2) I agree that adapting Watchmen into prose -- while incorporating all the relevant background details -- would result in a mess. However, I doubt that it would be as long as the over-a-thousand-pages LOTR books.
Besides, Snyder isn't adapting a thousand pages -- he's adapting... what, 240 pages? And all the imagery is already there. Sure, he'll have to cut some stuff out, but still.
I guess what I'm trying to say is that adapting LOTR to fit a 9-hour movie couldn't have been any easier or more difficult than adapting Watchmen to fit a 2-hour movie.