"At GDC09, we got to talk with many people and listened to a variety of panels on all aspects of game design. But there was little doubt as to which panel attracted loads of attention -- the Jeff Kaplan panel on quest design in World of Warcraft.
Warcraft has come extremely far in terms of their UI design and quest implementation since the game launched back in 2004. Just between 2007 and 2009, Kaplan revealed that over 8,570,222,436 quests have been completed, while the daily average was 16,641,409. With those numbers in place, it's safe to say that World of Warcraft players are driven by their questing."