Co-author of the first MUD, Richard Bartle, gave an interesting keynote address at the Independent MMO Game Developers Conference 3.0 in Las Vegas earlier this week. In case you missed it, Ron Meiners of the Virtual Cultures blog has a very handy summary.
Bartle's speech, titled Pleasing the Teller, groups MMO players into three categories, eloquently characterized as three lost girls from popular literature. Alice, from Alice in Wonderland, prefers to go wherever flight or fancy takes her. Alice appreciates balanced (sandbox) MMOs like EVE. Dorothy, from The Wizard of Oz, has a one-track mind and prefers to follow the yellow brick road. Dorothy prefers MMOs on rails like WOW. Finally, Wendy from Peter Pan is a story-teller and wants to create experiences for others. Wendy prefers virtual worlds like SL.
To craft the ideal MMO, Bartle suggests designing it to allow space where all three player-types can intermingle. How can this be done? Read his presentation to find the answer! His parting words are "How things were doesn't mean that's how things should be," which is interesting since he is basically calling for a throwback to MUDs before these gaming philosophies diverged in the first place.