Conflict. Doesn't matter if it's social, physical, or psychological, but side A needs to be at odds with side B, and have PC's on at least one of the sides.
It's even more fun if you can somehow end up with PCs on two or more of the sides!
A few things I've learned about what makes a typical dungeon crawl adventure more interesting from the nuts-and-bolts design side:
- if there is more than one vertical level to the complex make sure there's numerous ways to get from level to level - lots of stairways/shafts/elevators/ramps/etc. - instead of the all-too-common single staircase
- if there are more than two vertical levels make sure at least one or two vertical accesses bypass at least one intervening level (preferably in a way that doesn't make it obvious)
- try to have at least several entrances from/exits to the outdoors, and best if one or two of which aren't immediately obvious
- try to avoid "linear" room layouts where possible as the only movement choice they provide is whether to go forward, go backward, or stop - closed loops are your friend*
- pit traps are fun but get old quite quickly; chute traps that safely deposit their victim(s) on a different level and cannot be climbed are better
- mazes might seem like (and often are!) fun from the design side but after the first one or two they get really dull from the player side unless the maze is integral to the story e.g. the goal of the adventure is to take down a particular minotaur which is known to live in a maze...
- have some of the occupants use the vertical levels to their advantage, even if it's as simple as kobolds dropping rocks through a hole in the ceiling
- without overdoing it, make sure that you both put in and describe features and dungeon dressing that are no more than just features and dungeon dressing - sometimes a 10' diameter pool of water is just a 10' diameter pool of water. Not everything has to be dangerous or a threat.
- build in some rewards or benefits that an astute, careful, or just plain lucky party can take advantage of if found e.g. a pool of foul-smelling muddy water that once per day per character (or even just once, period) gives best-quality night vision for an hour if a cupful is consumed within a minute of removal from the pool - in my experience players love these sort of things
* - great examples in print to illustrate my first four points are L1
Secret of Bone Hill and the Judges Guild adventure
Dark Tower.
Lanefan