Paul over at blogofholding.com posted this about a one-monster dungeon, like Theseus vs. the Minotaur or most pulp stories. You know, how Conan isn't hacking down hordes of orcs, he's fighting the one big lizard or ancient god or whatever in the place.
I posted a response on his blog but I realized I didn't want to lose that comment in the ether of "Where did I write that?" So here it is again, slightly annotated and hyperlinked.
Said I, considering a dungeon with one big bad monster:
"If it's possible to explore the dungeon, find the Big Treasure, and sneak back out with getting whacked by the big monster, sure, tension. It adds a whole new dimension. As long as the monster is scary enough that avoiding it would be better, and there is a non-monster-slaying goal involved (escape the dungeon, find the treasure, get through the maze into the secret lair of whomever, etc.) So the monster is the worst thing that could happen, and it's not something that helps you with your goal.
Theseus is a bad example of that, though. The minotaur is the quest item (find him, kill him) and escaping the maze is the tough bit. The maze is the monster there, if you follow me. A better example might be the sea maze in front of Imrryr, where sea raiders hope to hell they get through the dungeon to the prize (sack the dreaming city!) without encountering the monster – the golden battle barges. Smaug's lair, too – Bilbo wants to find the treasure, not really find the dragon. Getting rid of the dragon is fine but everyone would rather not be the one to fight it.
But it's just a big maze I have to explore and hope I find the monster I want to kill . . . eh, that doesn't sound too fun."
That's pretty much it - if the monster is the goal, an empty maze to navigate to find it doesn't sound fun. Not that it wouldn't be a cool story (I like the Theseus story myself) but it's not as interesting to spend 3-5 gaming hours at the table hoping you finally find the bloody minotaur. It's way more interesting to spend 3-5 hours thinking please-please-please-please-don't-let-this-room-have-the-minotaur-I-just-want-to-find-magic-amulet-and-go-home.