"To play the game, you put currency into the machine. You then pull the knob and wait for the result. When the result is presented, you are rewarded with a cacophony of exciting sounds, attention-grabbing images, and some form of currency. Often times, this winning helps you progress towards a larger goal. You also have the opportunity with each play to win a rare prize of significantly higher value than the value of…
WoW still has very well designed game mechanics that are fun on their own. Learning (ie advancing up the learning curve) and decision making play a big role in having fun in RPG's. These are also true game mechanics (meaning they're fun without reward mechanisms).
On one end of the spectrum would be a game like Team Fortress 2, which is fun because of it's mechanics; on the other end of the spectrum is a Zynga game that is fun for the reward mechanisms. WoW, and other RPG's would be somewhere in the middle.