What makes the questing fun or not is all in how the story is told. A good quest has a real / believeable situation that needs resolving. e.g. if some heavily armed guard is asking you to kill a bunch of ferrets, that doesn't make any sense because the guard could just do it himself. A good quest has multiple steps with varying goals. A good quest (IMO) encourages some kind of interaction with other players (not necessarily in the form of partying). In an MMO, a good quest is one that isn't exactly the same for every person that does it, and is only available at certain times or under certain conditions (e.g. to get rid of that feeling you get when an NPC that sits in the same spot 24/7 is asking everyone to kill the same 10 mobs regardless of when or who talks to him) that make logical sense.

Basically, anything that minimizes the potential repetitiveness of quests in general.