If D&D is a unique genre, what are it's characteristics, what defines it?
Primarily, its a mash-up of a bunch of different elements of fantasy, myth, and hints of sci-fi and horror all rolled up into a "kitchen sink" of ideas, which the DM can use as many or as little of as he wants. Its the genre were feudal samurai, Celtic druids, Arthurian paladins, knights-templar, Tolkien hobbits, and thewy barbarians all join together in taverns to fight Hammer-horror vampires, Chuthulu-inspired aberrations, Greek medusae and whatever Chinese toy Gary bought in a Five-n-Dime.
Specifically though, if you want some of the genre tropes...
* Arcane/Divine Magic split. Arcanists use magic to attack, Priests use magic to heal.
* Magic is not good or evil; it exists. Heroes and villains can use it alike.
* Arcanists are weak, frail, and bookish.
* Rest to recover magic, or some other limit on magic use per day.
* There a hundreds of sentient "near human" races, each with one or more traits exaggerated to create a "niche" for the race. (Elves are graceful and wise, hobbits small and plucky, orcs brutish and strong, etc).
* The Four-man party: Warrior, thief, arcanist, priest.
* Dungeons as underground fun-house of monsters, traps, mysteries, and mazes, often with more emphasis on player-challenge than on any given purpose.
* Color-coded elemental-based dragons
* Magic items Galore, to the point of characters being overladen with them
I'm sure there are more.