If I were to play a paid MMO, it'd probably be WoW, the story and such appeal to me.

Be that as it may, I'm reminded of the monkey with button experiment that some dude at Blizzard apparently quoted awhile back.

If you rig a button to dispense a treat when pressed, a monkey will push it for awhile before getting bored and walking away.

If you rig a button to dispense a treat every 5 presses, the monkey will push it for even longer, but will eventually get bored and walk away.

If you rig the button to have a 5% chance to dispense a treat whenever it is pushed, the monkey will press the button for HOURS.