While I'm no longer a big comic book geek, I damn sure was in the 80's and early 90s. In fact, I was the manager of a comic book store when I was in Jr. College in 90/91. And yeah, I read the Watchmen when it came out in 86/87--one issue per month, for just over a year.
I can't stress enough how BIG of a deal the Watchmen was in 1987. Think about how much Watchmen stuff as you're seeing now, and then multiply that times about a million--that might adequately illustrate how "overexposed" the Watchmen was to the comic book world at that time. You couldn't walk near a comic book store without someone bludgeoning you over the head with the latest issue of The Watchmen.
Make no mistake, The Watchmen is absolutely the greatest comic book/graphic novel ever put together by anyone--but NOT because of the story/characters.
You see, The Watchmen completely played to the strengths of the comic medium in almost a textbook fashion. The methods the panels were laid out, the narrative technique, the graphic repetition, the visual allusions, the sideline histories that indirectly flesh out the main story, and of course the story-within-a-story that foretold not in plot but in emotion what the (then unknown) villain was experiencing.
On top of all that, it painted a vivid picture with deep characterization that was a dark reflection of our world in 1987, and dammit, it did so with a cast of completely unknown characters. That was just unheard of in a world being spoonfed simple beat-em-up stories by known good guys like the X-Men and Superman and Spider-Man and Bat Man and The Hulk.
The Watchmen also proved that comic books could be "adult" oriented and be commercially successful. Prior to 1987 and The Watchmen, adult oriented comics were just a small subset of the comic book world. After 1987 and The Watchmen, the comic book world was PRIMARILY adult oriented--and the comic industry was never, ever the same afterwords. No longer a "kiddie book" industry, comics were now able to (in fact, expected to) cover grown up, adult issues in a mature manner.
There were also a lot of marketing things the Watchmen (along with Frank Miller's "The Dark Knight Returns" that came out around the same time) did that radically changed the comic book industry-including making a viable commercial product out of a one-shot story--meaning, a story with a beginning/middle/end, as opposed to a continuous and never ending monthly super-hero story. That in turn created the collected "graphic novel" as we know it today. It also made the 2-to-3-times-as-expensive "prestige format" comic book (glossy paper, thick cover, perfect bound) a wild commercial success. The crappy newsprint comic books we all grew up with were no longer good enough for the adult comic book consumer...not after The Watchmen.
So you see, The Watchmen is/was the greatest comic book/graphic novel of all time not because of the story (I mean come on, it's just a simple "whodunnit?" plot with a crazy twist for an ending) but because of just HOW BRILLIANTLY the story was told, and also the ramifications it had on the comic book industry.
The movie, unfortunately, is just going to be an adaptation of the story. I'm sure it'll be a damn-fine adaptation (Zach Snyder's proven he can do that with 300) but it won't be able to do much more than capture a fragment of the original storytelling.
<sigh>
Because in the end, the story isn't what really matters about The Watchmen.
It's HOW THE STORY WAS TOLD that matters.