I've played a moderate number of CRPGs and PnP RPGs, and in all that time, only one dungeon complex has ever really stood out to me: Watcher's Keep. It was the megadungeon added to Baldur's Gate 2 with the Throne of Bhall expansion, and it was incredible: tough fights and good loot, sure, but even Diablo has that stuff.
What made it awesome were the unique moments that the dungeon contained (the amazing Zork throwback, the hall of statues, the ghosts, the seal guardians, the laboratories, the guy in the cage, the machine of Lum the Mad, the sneaky priests that guarded it, and ... of course ... the Demogorgon). I've never experienced a dungeon like it.
If the designers can capture the feel of Watcher's Keep -- a series of amazing RPG experiences (puzzles, characters, tactics, moral choices, mystery, treasure, danger), loosely connected -- I'll be delighted.
How about you? Any amazing dungeons in your past that Endless Paths should be inspired by? Pen and paper are good, too! I'm not asking about mechanics so much as about feel.