Quest based XP is a new concept in Dungeons & Dragons Fourth Edition. The concept is simple: The players will acquire certain quest cards from the DM. Some cards may be related to the character's back story (e.g., "I must kill the one-eyed dwarf who took advantage of my sister"). Some cards will be based off of interaction with NPCs (e.g., "The elders of Barrow's Edge want us to put an end to the goblin menace"). Some quest cards may even be treasure (e.g., "Look, this map says there is great treasure in some place called 'The Tomb of Horrors'. How bad could it be?").
When I first heard about the concept, I have to admit I didn't like it. Handled poorly, I can see quest cards turning D&D into a game of Oblivion where PC's talk to random NPC's until they get a quest, then they quickly run off and do the quest. Nevertheless, handled well I believe quest based XP can enhance the game.
One good thing I can see coming out of quest based XP is its potential to bring details of the player character's backgrounds into the game. Too many times I have seen players write up backgrounds for their characters that never come up during gameplay. Having a couple of quest cards lying around related to the character's background gives the player an incentive to bring it up during the game.
Quest cards might also encourage adventures that don't involve killing every creature in the dungeon. Here is an example of an actual conversation with one of my players:
Player: "If you sneak past or talk to the monsters, you don't get XP for them."
Me: "That's not necessarily true. I usually give half XP for bypassed monsters and give bonus XP for creative solutions to problems"
Player: "But then how much XP you give out is up to you. I would rather kill the monsters because the book specifically lists how much XP you get.
I chose not to tell him I rarely paid strict attention to Challenge Ratings and often "winged it" when assigning XP at the end of the adventure. However, this is a case where quest cards might have helped out. A quest card that said "Steal the the king's ruby without alerting the palace guards" would encourage stealth. Or a lot of assassinations. In any case it would discourage charging the gate.
In the end, I guess I came to realize quest cards are just another tool in the DM's arsenal. Misused, they have the potential to screw up a game. But used properly they can enhance the D&D experience.
Plus, I like playing Oblivion, don't you?