Well look at the pen and paper role play games that promote actual role play as opposed to leveling. Vampire (the p&p game) gives out points to the best roleplayer and the best actor each session. There''s no real xp on offer for those who just slaughter mindlessly (unless that _is_ their character) only for those who roleplay, act their character and learn new things each story.
To allow this you need some way of determining who has done the best roleplaying which means either having a permanent ref watching each party (possible in LAN games, impossible in On-line games) or have people nominated for role playing by fellow players (hugely open to abuse).

The only way to get roleplaying into roleplaying games is to take the leveling players out of them. The only way to do this is to remove the levels and only leave in that which defines character.

A skill based system would be a start because what you can do helps define your character and promotes roleplaying. With a skill based system you are defined as more than just a 34th level warrior, you are more of an individual.

At one extreme you could remove all concept of experience completely and have characters who only change in terms of their personality. Their stats remain static. That would almost guarentee to attract roleplaying players. But I doubt there''s enough of them interested to be able to call it massively multiplayer.

Out of that barrage of meandering thought I think that having small parties with refs on LAN system will be the only possible solution.

Off the top of my head that is :-)

Mike