When I was browsing through the rec.games.roguelike.development newsgroup I
found a thread concerning quests. Below is the essential parts of the thread:  

THE ADVENTURER
- Reach a certain location
- Map a location and bring the map to a person or location
- Find n objects and bring them to a person or location
- Find or catch n creatures and bring them to a person or location
- Something strange is going on. Track the source and stop it
- Escape from the trap (dungeon)  

THE HERO
- Get n followers (of a specific type)
- Gather forces (beings and/or objects) and lead/bring them to a person or location  

THE WARRIOR
- Kill n monsters (of a certain type)
- Clear a location from monsters  

THE MERCHANT
- Manage a business to get beyond a certain limit (time/money)  

COMPETING WITH THE ENEMY/ARCHENEMY
- Reach a location before the archenemy
- Get an object from a person before the archenemy
- Get an object from a location before the archenemy
- Find an object before the archenemy
- Steal an object from the enemy  

PROTECT FROM THE ENEMY/ARCHENEMY
- Protect a location from the enemy (until civilians have escaped)
- Protect a person from the enemy
- Find and protect a person from the enemy
- Guide/protect a person on the way from one location to another  

Try to vary the number of ways in which the PC can enter into quests. It can be
so much more interesting to be travelling the countryside and hear a maiden's
wails than to be told exactly what the quest is & where to go, etc. by some
'questmonger' in town.