I have never found anything mechanically addictive in WoW. There are single player games that essentialy work the same way, but you don't see people ruining their lives over them. What people get addicted to is the social aspect, and the raiding culture nurtured by the players. I don't think I've ever run across a typical "WoW addict" who was into something else than the raiding part of the game.
Human beings generally care about how they are percieved by others, so that's the angle you'd want to look into when explaining WoW addiction. Some people may not get enough positive reinforcement in real life, and they feel more valued as persons by how their guild mates treat them. I think the whole thing about comparing epics to the chemical rush you get from heroin which you often see in the mainstream media is completely ridiculous, a waste of time and is looking at the issue from the wrong perspective.