Well now that I've been playing D&D for over a decade, I can safely say Nostalgia has a big part of it. The clatter of dice on the table, the smell of beer and pretzels, pushing painted (and unpainted) figures around my well-loved vinyl battlemap, worn and stained from a decade of battles. The whole experience is comforting, like a warm blanket.
Nostalgia aside, I really like the basic D20 system. D20 + Mod is simple and gratifying to me, even if there are better options (I think BRP is incredibly elegant and crunchy simultaneously.) Ability Scores and Classes, Feats and Spells, all of that stuff is fun to play with, even if its flawed, unbalanced or confusing. I just like it.
Similarly I enjoy the genre of D&D. I don't mean Fantasy, I mean D&D as a genre. "Dungeon Fantasy" I've seen it called sometimes, but that specific Milieu of fantasy inspirations that melt together to form something weird but familiar. The classes and races we've all come to love, going up against classic monsters like goblins and dragons, before descending to fight uniquely D&D monsters like Beholders and Mind Flayers. Its the result of decades of D&D campaigns creating an identity for the game that no other game has, and persists mostly regardless of setting.
There are lots of other fantasy RPGs out there, some might even be better, but nothing does D&D as good as D&D does it. (Pathfinder too, but they're basically different flavors of the same thing, IMO.)
