Guardian has a fun interview with MUD creator Richard Bartle.
My stance on MMORPGs: The fun is in the fluctuation from the baseline, but in my experience the more you play the closer you approach that line and the fun disappears into the baseline meta-MMORPG.
That said, you can draw out the life of the fluctuation with the social aspect for something approaching infinity. I am probably different from 95% of players out there, but I would just rather move into a new game at that point.
(I am my own avatar, if the big thinking -fluctuation/fun- part of the game is over, I move on instead of concentrating on incremental dominance for an e-avatar.)