I like finding +1 items, they’re real workhorses!
– Blair 2010-02-06 00:48 UTC
Related links:
A nice collection! 
Update:
Interesting additions by another blogger:
- a secret that the players can figure out about the dungeon itself, or some similar concept.
- interesting terrain to make things more memorable and more challenging.
Update:
2012-11-17 How To Build A Dungeon
– AlexSchroeder