World of Warcraft is an addictive game because it feeds into people's subconscious egos by using a system of gradually more difficult challenges rewarded by a disproportionately higher system of return.
For example, there are numerous mini challenges throughout the game, and as you accomplish them, the rewards are often higher than the risk.
If you kill numerous small enemies, there is a challenge to your character, but only superficially. You, as a person, receive actual feelings of accomplishment and pleasure by completing tasks and winning battles.
The game then gets into higher difficulties and larger rewards, and even elements that require team play and strategies. These things again feed into individuals' ego subconsciously; even if you have a fine normal life with a stable job, good income, reliable friends and family, the pull of constant reaffirmation and reward that the game gives you is very strong.
This doesn't even touch on the social aspects and the personalization aspects of the game. Every major holiday sees special activities and challenges to complete, a surge in player traffic (so there are more people to play with), and other things.
You can customize your character exactly how you'd like, and customization is a very addictive thing, going all the way back to stickers on binders in school and dolls and action figures as kids.
You can play the game free to level 20, at which point you get a customer horse or zebra or ostrich; I recommend you try it to find out exactly why it's so addictive.