From an administration perspective, one of the most difficult things about running World of Warcraft is managing the economy.
In WoW the economy is a very delicate balance. The currency of the game is earned very slowly but by millions of players at a time. In order to balance the economy the game has built in money sinks. The most famous of these is the cost of flight. (There was a widely reported indecent of a woman trading sex for the cost of her flying mount.) But there have always been various money sinks in the game to try to keep inflation under control.
But the latest expansion introduced the most brilliant money sink. Historically money sinks have been a reward system. You sink x amount of money, you get y as a reward. Five thousand gold gets you flight training. But what the developers needed was a way to take money out of the game without any rewards given. And they came up with the "Achievement" system.
With achievements, the game tracks the tasks you have to complete, and when you complete the task it gives you "achievement points." And that's it. The achievement points aren't worth anything. You can't spend them on items or rewards of any kind. It's just a tracking system. You basically get a "check mark" saying you have completed that task.
And the players have gone crazy for the achievements. They are spending hours upon days upon weeks completing these tasks for literally zero rewards. They spend all their resources until they aren't even able to participate in actual gameplay because they've exhausted themselves chasing achievements.
It appeals to the obsessive compulsive nature of all people. People are very naturally "box-checkers" and love to complete things. They have to complete their collection of everything. Without it they feel incomplete. They feel a nearly physical need to finish things. It's amazing to watch.
I'm just not a box-checker. I don't feel the need to finish something "just to finish" it. If there is a reward for completing a task I feel a sense of accomplishment, but if I complete something just to have done it I usually feel disappointed.
When doing achievements in WoW I begin to feel used and manipulated. I rarely try to complete them intentionally, and when I do I feel almost ashamed.
I still enjoy the game, and I have actually built a respectable player up. I'm the 32nd ranked resto druid on the realm I play, which is really not bad. I can keep up with the best players in the game, so I really feel like I can contribute to a group when I play. I take a great deal of pleasure in being an instrumental part of a raid, helping the group achieve a particularly difficult goal (as long as that goal includes a reward that improves the group.)
I guess I'm not very OCD, and if you have a problem with OCD I wouldn't recommend World of Warcraft. But if you just really get off on checking boxes, it may be the game for you.