My guild, like most, has seen players quit the game for a variety of reasons. Sometimes it is another game stealing their gamer hearts, and equally often it can be a real-world time constraint that is given priority. What interests me is how the majority of these players return eventually. Most of the time, this seems to happen when their real-world issues get resolved or when they discover that other MMO just doesn't love them as much as WoW did. Nearly all of our regular guildmates who have "given up WoW," including selling or deleting accounts, have returned.
I had even "quit" WoW for nearly a year and let my subscription go. Just before I came back, I had been playing Fallout 3 and beat it, completing pretty much everything. When it ended, I wanted a game that I could keep playing, a place where my character could keep growing and the world wouldn't end when I beat some random boss. I instantly remembered WoW, bought Wrath of the Lich King, and have been playing strong ever since.
If you or someone you know has ever made the decision to give up the game and move on -- more than just taking a break -- what has brought you or your friend or guildmate back into the world? Is it a realization that WoW just might be the best MMO on the market today? Is it a recreation of spare time for gaming? Or is it something else even more interesting?