Everytime I play WoW I think that it is worse for me than all the cigarettes, pot and everything else I've put in my body. At least with the other stuff I still get out and do shit, when I play I become rooted to my couch unable to move for hours on end.
Cracked actually did a great article on how MMO's keep you hooked. The tricks they use to manipulate people are so simple but so effective.