The
Power of Habit by Charles Duhigg.
Habits actually consist of three parts:
The triggers are all those moments in our day we come to associate with logging into WoW: waking up, morning coffee, getting home from work, raid time, an invasion is up, flipping table missions... The frequency varies with the player and time available for gaming, but anyone playing enough to be thought of as an addict has made these mental associations.
The activity is the game.
Rewards include social interaction, a sense of power and progression that may be lacking in real life ("I may not have even applied for a new job/promotion this week, but I topped the meters in my tryout for that raid team!"), and freedom from some of the concerns that haunt us in the outside world.
WoW is certainly habit forming, but is it an addiction? We can quit any time, right?
Seriously, I would suggest drawing the line where the game significantly interferes with real life priorities like family, friends, and livelihood. My other biggest concern with WoW is the sedentary lifestyle. WoW players complain about making their cartoons run; how many of them get out for a run themselves?