In terms of both creativity and commercial standing, it is almost impossible to overstate the importance of Watchmen; critically, the book won multiple awards both within and outside of the comic industry, including a Hugo Award in 1988 in the "Other Forms" category. It has been consistently cited as one of the first works to demonstrate the maturity of comic books in terms of content and medium, and transcended both to be placed on Best Novel lists from Time, Entertainment Weekly and the Wall Street Journal. Commercially, the collected edition has remained in print constantly since its first release - a fact that upsets Moore, whose contract with DC asserts that the rights to the series will revert back to he and Gibbons should the book fall out of print for a specified amount of time - with multiple editions available to suit every price range (including, now, a "motion comic" animated version to accompany the movie release), and the book has consistently been one of the more successful collected editions for DC Comics, annually making it into the upper echelons of the sales chart despite its age.
Joss Whedon has said that Watchmen is "proof of everything a comic could do, but also an affirmation of everything comics had done," which is a good way to explain, in one line, one portion of the significance of the book within the medium. Moore and Gibbons' deliberate intention to produce something "designed to show off [comic book techniques] things that other media can't [duplicate]" resulted in a book that is almost technically perfect as an exploration of a medium, despite what you may feel about the actual plot (In fact, Gibbons has since admitted that the plot "just really isn't the most interesting thing about Watchmen. As we actually came to tell the tale, [the way we told the story is] where the real creativity came in"). In terms of ambition, world-building and technical excellence, no superhero series before or since has come close to matching what Watchmen has accomplished.
In tandem with The Dark Knight Returns, Watchmen also (accidentally) redefined the notion of how dark a superhero story should go, leading to a spate of increasingly dour, quasi-deconstructionist approaches to the genre over the decade or so after its release; even recently, comics like Mark Millar's Ultimates and J. Michael Straczynski's Supreme Power have born an unmistakable influence from the series, ignoring Moore's own attempts to lighten the genre with books like Top 10 and Supreme. Entertainment Weekly's Jeff Jensen has said that he feels that Watchmen is such a landmark in terms of its impact and influence that comic eras should simply be "Before Watchmen" and "After Watchmen." Surprisingly, such hyperbole may be well-earned.