To begin with, I have not RTFA (damn you websense!) nor have I ever played WoW. But I can give some examples of addictive games that I have played, such as Sid Meier's Pirates! As many other posters have pointed out, one of the features that they feel makes WoW addictive is the social interaction. This has no appeal for me, which is why MMORPG's haven't ever really captivated me the way most single player games have. I think that the addictiveness has to take into account both the nature of the players and also the player's goals. In the case of Pirates! the game became very addicting because the quests that would be given to you could be accomplished in about 20 minutes each. This, coupled with a fairly intuitive control scheme using just the number pad on the keyboard made for a great pick up and play kind of game, but always having the ability to complete a particular quest in 20 minutes or less triggers a "just one more" kind of play style where I lose track of time and next thing I know I've been playing for 14 hours with minimal breaks. Having short, attainable goals that tie into a long term goal provides a sense of accomplishment that produces a desire to keep on playing without interruption.