If I might be so bold as to offer an idea, rather than tying every cool monster kill to a quest have the monsters just be there. The player wanders upon them and decides to kill them, maybe it doesn't give XP explicitly (since that seems what you are trying to avoid), but instead you take the monster's head or whatever and THAT starts a quest to take the head to an NPC for a reward of exp and whatever else.
Go even further, have the NPC offering the quest, but if the player isn't on the quest because they hadn't met the quest giver yet THEN the monster drops the item to start the quest.
Also needs to be some sort of punishment for killing innocent people, especially after completing a quest for them, without ruining the possibility of playing an evil character.