The challenge. I'm not talking about hack'n'slash, or how difficult that combat encounter was. I'm talking about challenging the PCs in more than just fighting. There have been some great answers here about not just fighting. Likewise, Dausuul is spot on by stating a dungeon is a framing device. If you treat a dungeon as a microcosm of an adventure/campaign then if you're world is fun, there's no reason the dungeon can't be also.
Throw some stupid encounters in there: the 'bad-ass' kobold who's "gonna mess you up!" Screw with the party by splitting them up and have one of the 'found' players play a doppelganger out to kill the party.
My favorite dungeon: Arenaworld!
A DM canceled on us at the last second, so my group asked me to create a one-session adventure. Not having anything particularly ready, I cheated and literally created the dungeon straight out of the DMG. I placed the group in a dungeon with a town in it where no one had memories of their past (incl. the PCs). Rather than stay in the boring town they went adventuring. Up they went to the level 1 dungeon. The end result was that the DM never showed up again and I continued the 'cheat', level by level.
My favorite dungeon episode:
A DM got tired of us ignoring his mazes by cutting through walls, so he made the dungeon invulnerable. We succeeded in taking a door of it's hinges and carting it around as an invulnerable tower shield. Came in quite handy.
Throw some stupid encounters in there: the 'bad-ass' kobold who's "gonna mess you up!" Screw with the party by splitting them up and have one of the 'found' players play a doppelganger out to kill the party.
My favorite dungeon: Arenaworld!
A DM canceled on us at the last second, so my group asked me to create a one-session adventure. Not having anything particularly ready, I cheated and literally created the dungeon straight out of the DMG. I placed the group in a dungeon with a town in it where no one had memories of their past (incl. the PCs). Rather than stay in the boring town they went adventuring. Up they went to the level 1 dungeon. The end result was that the DM never showed up again and I continued the 'cheat', level by level.
My favorite dungeon episode:
A DM got tired of us ignoring his mazes by cutting through walls, so he made the dungeon invulnerable. We succeeded in taking a door of it's hinges and carting it around as an invulnerable tower shield. Came in quite handy.