I like the built-in goal (via the XP/level system) where it's intuitively understood that the players want to explore dungeons and wildernesses, fight monsters, and recover as much treasure as possible. Other rpgs jump through hoops trying to provide realistic motivations for characters to go on "adventures" but in D&D none of that's necessary because of the way the rules are set up -- you go on adventures because that's what's done; it's how the game works.
I like the fact that a player can get started in about 5 minutes -- 6 ability scores, a class, a race, and an alignment and you've got a pretty good idea what your character is like and are ready to go. Other games that don't rely on archetypes and create more realistic and well-rounded characters are great for advanced/dedicated players, but when you've got a bunch of casual players and newbies, the shorter the delay between sitting down at the table and the actual fun part of play (exploration, role-playing, problem-solving) the better.
I like the kitchen-sink approach to fantasy -- the fact that no matter what your approach to fantasy is, it's in there somewhere. D&D sucks at accurately recreating any one specific pre-existing fantasy world, but is unparalleled in recreating all of them at once and making a fun game out of it that everyone can enjoy, not just the Tolkein-nuts or the Conan-nuts or the Arthurian-nuts or the medieval history buffs, etc.
I like that D&D recognizes that it's all a game and doesn't take itself too seriously -- it's not about creating art, it's about pastiche and borderline parody, a bunch of guys with a mutual interest sitting around a table having fun together -- competing and trying to "win" but also cracking jokes, making puns, and going off on wild tangents. If I want "serious" fantasy (intricate plots, fully developed characters, a consistent and believable alternate world) I'll read a book. Too many other rpgs take themselves way too seriously; D&D is refreshingly unpretentious.