I suspect a lot of you out there will be saying, "Who?" There are a lot of tributes to the man out there on the web today, written by people who actually knew him. It won't take much searching to find out about Schwartz, whose friends called him "Julie". Here's a good starting point.
Though I know of his career as a literary agent in the Pulp Era, helping to shape the careers of Ray Bradbury, Robert Bloch, and Alfred Bester, and to bring wider attention to the works of H. P. Lovecraft, </i>I tend to think of Julie as The Father Of The Silver Age of Comics. As an editor for DC (National Periodicals) in the '50s, '60s, and '70s, he revamped The Flash, Green Lantern, The Atom, Hawkman, and oversaw the creation of the Justice League of America. He also saved Batman from cancellation in the mid-'60s, re-emphasizing the character as a detective and veering sharply away from the silly science-fantasy morass he'd been sinking in since the '50s.
In addition to his work at DC/National, he was indirectly responsible for the Marvel renaissance of the 1960s. National's success with Justice League of America prompted Martin Goodman to order Lee and Kirby to come up with their own version of the title -- a move which gave us The Fantastic Four, and all that followed.
I've often invoked Schwartz's name to describe a certain distinctive style of comic book writing: characters who solve problems through a combination of logic and creativity, who continually improvise new uses for their powers, and frequently invoke obscure scientific factoids in the process. It was a style that worked equally well with the down-to-earth detective stories of the mid-Sixties Batman and the wild, neo-Barsoomian space opera of Adam Strange. Sure, it was frequently doubletalk, but a Schwartz-edited title was never just a simple slugfest.
Schwartz books would frequently include panels or pages apart from the main story, imparting some interesting big of scientific trivia -- most notably in the "Flash Facts" boxes in the Scarlet Speedster's title, but elsewhere as well. This wasn't just to make the books "educational" -- it was because Schwartz, part of SF's Golden Age, was a hard-core geek in the most positive sense of the word. He delighted in this stuff, and by enthusiasticly sharing tidbits about the Doppler effect and weather patterns and dwarf stars, managed to make that delight infectious.
All told, while names like Tolkien and Asimov and even Lee and Kirby might be better-known, I think that Julius Schwartz was more influential -- not just within the rarified circles of SF and comics fandom, but across the spectrum of 20th century culture.