What makes D&D, well, D&D? Is it the d20? Is it elves, dwarves and halflings? Is it fireball-slinging wizards, curing clerics and vancian magic? Is it multi-hued dragons and the other bizarre monsters? Is it random 3-18 stats and stat prerequisites? Is it alignment? Is it dungeons filled with monster menageries and improbable death traps? Is it the Middle-Age-Greyhawk-esque milieu?
It’s impossible to say exactly what defines D&D. But I think that one of its traits not only helps define D&D, but makes it the best-selling rpg for the same reason that the Bible is the best-selling book in America:
It has something for everyone. D&D has human fighters for the farmboy-done-good experience, it has elven wizards for the mysterious outsider experience, it has hobgoblin blackguards for the fascist militant experience, it has centaurs for the some-old-wizard’s-experiment experience, it has war forged for the existential dilemma experience, it has lizard people for the just-plain-weird experience. The list goes on and on. And yes, it’s utterly absurd, but that’s the beauty of D&D.
Theoretically, D&D lets you play anything fantastical up to and including the kitchen sink. That’s not to say that cutting back on options is badwrongfun; I once ran a short Creation campaign, which is a human-only world. But cutting down on options is cutting back on, possibly, D&D’s greatest strength.