Not too long ago I got in a bit of a tiff on someone's blog (Kalinara? Ragnell? Someone I check frequently) with one of the guys from the excellent 4thletter blog, by inadvertently calling Marvel fans immature (long story), and suggesting that DC was generally more literary. At least I wasn't making a joke about the President and the troops (zing!).
Anyway, a question occurred to me the other day: does Marvel have an equivalent of Sandman? How about Alan Moore's Swamp Thing? See, despite the editorial separation of Vertigo and the mainstream DCU, the membrane 'twixt the two is at least semi-permeable. Swamp Thing was instrumental in the death of Zatara, Zatanna dated John Constantine, Martian Manhunter met Morpheus (in the moonlight!) and Dr. Destiny used Dream's crystal as the source of his power, and then there's Animal Man, who jumps back and forth more than almost anyone. Almost, because Phantom Stranger, Deadman, and Etrigan the Demon have a tendency to pop up in both 'universes' almost equally.
So, while the DCU wheeled about and did their things, early Vertigo comics bolstered the mainstream universe by giving it a distinct and deep mythology, developed through some of the best comics ever written.
And, so far as I know, Marvel really doesn't have an equivalent. There are great Marvel stories to be sure--The Death of Captain Marvel, Squadron Supreme, etc--but few are really tied into the fabric of the universe. I can't think of any that have the same scope, the same intent, as the Moore and Gaiman and Morrison work at DC. And maybe this is where I made my blunder...to me, one of the best things about DC is the tapestry, the universe as an entity, with all its history and mythology and epic scope. Marvel's greatest strength, on the other hand, is in great single stories that build on character tapestries. You could cut Spider-Man off from the rest of the superhero community, and you wouldn't lose much (these days, you'd gain quite a bit), because Spider-Man's strength is in his personal history, not in his relationship with the rest of the Marvel universe. The same could be said with any single hero or small team--break the Fantastic Four away from continuity, or the Avengers, or the X-Men, and just tell stories about their characters (and rogues) for a year, and the stories really wouldn't suffer. In DC, characters are defined by their relationships to other superheroes as much as they're defined by their own lives and supporting casts. Imagine a DC without the World's Finest team, or the Trinity, or the Brave and the Bold team, or GL/GA, or GA/Hawkman, or GA/Black Canary. Suddenly, it's a much less interesting universe. Marvel's strength is in the individual; DC's is in the sum of its parts.