Fascinating, thought provoking stuff on the nature of the MMO phenomena:
What is it, then, that convinces a subscriber to pay triple digits every year for a single game? What facet of the game would cause the whole tower to crumble if removed? The answer is persistent character progression. Imagine that World of Warcraft is now back to hosting thousands of players on each server (plus several hundred in the login queue, of course).
This time, however, when a player gains a level, it only lasts until the player logs off—like a game of Quake, where all kill stats reset when the game is over. The same goes for abilities, items, and all other forms of progression. Players can still interact with thousands of other players and do everything else they could do before; the one change is that their character progress is no longer persistent.
My hunch is that under these conditions, the game would not have many subscribers left, as neither the gameplay mechanics nor the ability to interact with thousands of players would provide enough appeal to retain them