I don't really consider it quitting, since I do plan to start playing again once Cataclysm comes out, or maybe a bit before, but since I started playing WoW I've taken 4 extended absences from the game, including the one I'm taking now.
First time was because I didn't know anyone that played at the time and wasn't that excited by it. But once I picked it up again (with friends) the breaks have been because real life was conflicting with raiding, and after already getting the Loremaster achievement, I didn't really want to be playing just to do dailies...