The grand daddy of all games, in terms of potential. The hardest to design, the most difficult to balance, and the hardest audience to please. This is no easy task. But let us examine for a moment some of the fundamental designs of such a gargantuan product.
What motivation do the players, whether in a group or alone, continue month after month to play the game? What makes them want to come back for more?
Now, the only MMO I have played extensively enough to understand all it's mechanics and design, is World of Warcraft. So you're going to see a lot of references to Blizzard's design, which may differ somewhat (if at all) from , and spark rebuttal, However I think that most MMOs are designed enough like WoW that I can make general statements that stretch across the market as a whole.
Ok, with the background out of the way, /rant on.