Awarding experience in a ORPG

Here's something I don't see discussed very often, most people typically just think there's only one ways of gaining experience. When you complete a task. However there's some questions I have to you guys regarding this setup.

Usually experience is rewarded in three ways:

• Killing a monster.
• Completing a quest.
  
• Performing a profession based task.

But, what about changing the way you gain experience all together? Say a quest doesn't have to be completed, and you can go...

Awarding experience in a ORPG