Natural 20's (and anything that has to do with rolling lots of odd-shaped multicolored dice).
The weird, WTF creatures: gorilla cockroaches, giant floating eyes, brain-eating humanoid octopuses, carnivorous jellies... What's not to like?
The iconic character of the whole: dungeons, clerics, elves, wizards, artifacts, spells, alignments...
The excuse it gives me to meet my friends and have some laughs with (sometimes at) them.
The shared experience when I talk with other geeks about our adventures in the Temple of Elemental Evil, Castle Ravenloft, Undermountain, Tomb of Horrors, Age of Worms, Tomb of Abysthor or Rise of the Runelords.