It's the whole thing where you just are dying to be the best at everything.
I was first addicted to starcraft, I literally wanted to be one of those guys who's cheered on in korea for his starcraft skills. I seriously got pretty fucking good, but could never compete with great Korean players.
WoW on the other hand, were some guys I knew from CS (I also got all competitve into that). I never really enjoyed RPGs online, I felt they had no skill, but when level races were involved, and taking down big raid bosses FIRST were involved, I was hooked. For once I could be at the top of a game! 1st pvp 60 in the world! I LOVED THE FEELING!
After that dies down, you get addicted to the community you got yourself involved in.... and since you've been playing that game practically all day, you've cut off all the other activities you were doing.... so in other words, it's depressing to play, and it's depressing NOT to play.
See when you (yes you willco!) play WoW, you play it as a gaming experience. You know you're just there to move your guy around, use some of the cool skills, interact with a few people, complete some quests, and whoop de doo that was a little fun. For some people they don't see the game that way.
Now the MAJORITY of the people who play, are just generally depressed people who get involved in guilds and try to achieve status in that... that is by FAR the majority... and they just play because all of their good friends are in the game. Personally I can relate to the friends part, but my major addiction was trying to literally be the absolute best at everything.
I'm a competitive gamer addict, which is why 2 weeks ago I started playing starcraft again, and literally had to uninstall it AGAIN because it was hurting my school a little. WoW is just a time consuming addiction.. and when you just think of the general picture of it.. it's just fucking distinguished mentally-challenged. There's no more level races, except for when expansion comes out, and I know that I can't buy expansion... because I'll simply fucking race to 70! There's just raids.. and once you've done all of that (first in the world kills or not). There's no fucking reason.
Personally I never cared for gear, honestly. I got whatever it was I needed to go into the raid and get the job done. I always opted to get all of my class's items last in the guild, because all I ever gave a fuck about was the kill, not some gay gear. Everyone ELSE cares about the gear ;P