Those of you who don't read comics probably haven't heard of Will Eisner, but his influence was immense, and echoes all over modern popular culture.
Eisner revolutionized the field twice. The first time started in the 1940's, when he created the ongoing called The Spirit. On the surface, this was nothing terribly unusual -- a weekly 8-page strip about a masked crimefighter. And to be fair, the first few years were unremarkable. But after WWII, Eisner began to use it as a springboard for a remarkable variety of little meditations about life. This wasn't a big superhero yarn: it was about finding the extraordinary stories in ordinary lives. Almost every week he would create new characters to weave in, and in those 8 pages he would define that character more richly than most writers can manage in years of a monthly comic (or in a 500-page novel). In those weekly strips, he proved that really *good* writing was compatible with the illustrated form, even when under the deadline doom. The Spirit ran for a decade or two; DC is currently collecting the complete run in hardcover form, and it is the only comic for which I'm willing to regularly spend hardcover prices.
The second revolution came, oh, about 25 years ago, when Eisner wrote A Contract With God. This was a collection of interwoven short stories about life in the Jewish tenements of his youth, and is often credited with establishing the modern concept of the graphic novel. After being off the radar of most fans for a number of years, this firmly cemented his central position, as he began to come out with books on a regular basis, often drawing from this same autobiographical well. Even in his 80's, his distinctive writing and art styles remained steady and undimmed, more consistent than most people half his age. With the benefit of perspective, his later work brims with a complex emotional honesty that is rare in any literature, moreso in comics.
Greatness is something you can really only judge historically, and he is the only comic book writer who I can unreservedly call "great" -- his body of work, spanning over 60 years, establishes it without question. There are a few modern writers who may be judged similarly when perspective allows, but Eisner was the first, and I doubt anyone will ever truly better him. The comic book equivalent of the Oscars are the Eisners, and it's for good reason: he is a worthy standard to compare everything else against.
He'll be missed, terribly. Greatness aside, he is one of my three or four favorite writers of all time, fun to read even at his most heartbreaking. His work may never have had quite the poetry of Gaiman or the structural brilliance of Moore, but his understanding of the human condition resonated better than anyone else has ever managed...