In his seminal 1996 paper on the motivation of players in mmo type games ("Hearts, Clubs, Diamonds, Spades: Players Who Suit Muds") Richard Bartle deduced that interaction between different player types meant that only certain types of game would result in a stable configuration. His four "stable" game configurations are: 1. Achiever / Killer dominated game, 2. Socialiser dominated game, 3. A carefully balanced game where all four player types have similar influence and 4. The degenerate case of a game with no players left.
In the few mmo's that I have played each of Bartle's stereotypes are well catered for in the early game. Achievers get the challenge of levelling up, Explorers have new content to discover, Socialisers have the fun of setting up new guilds and the games have always had some pvp element to provide entertainment for the killers.
As the games have progressed however and in particular as players have matured into end game certain Bartle stereotypes have been catered for to the exclusion of others. WOW's endgame favours achievers above all. Guild wars favours killers with a nod in the direction of explorers due to the complex evolving meta-game. Eve is probably closest to a pure Bartle configuration and favours killers and achievers in pretty much equal measure.
I have to say I find this somewhat demoralising. I am a firm believer in diversity and I think the mix of player types and game-play styles really adds to a game. I want to play in a game that conforms to type 3 on Bartle's list. I haven't played enough of Lotro to foresee where it is going to end up. I hope they don't slavishly follow the WOW road and turn into an achiever focussed raid fest. The continuous drip drip of new content gives me hope though that this will not be the case.
By the way the database of over 300,000 Bartle test respondents at Guild Cafe shows Explorers in the lead at 33% followed by achievers (26%), Killers (22%) and socialisers(20%). This appears to completely contradict Bartles position that "few people are, by nature, explorers". Apologies for deep linking to that Guild Cafe chart but I cannot find the official way to get to it from their site.
In case you are wondering I did take the test and I am rated ESAK (Achiever 40.00%, Explorer 80.00%, Killer 26.67%, Socializer 53.33%). Apparently the most poular game for people like me is Entropia Universe. God Help me.