The generally excellent Extra Credits series occasionally touches on MMORPGs and their various systems has a pair of new videos out that might be of interest to you.
The first explores Bartle’s Taxonomy, which you might know as Bartle’s Test or Bartle’s Gamer Types. The video traces how Bartle came up with the categories and how those categories define how and why we play. The second tackles the application of these types of players and the challenge of balancing an MMO ecosystem to meet all of their needs.
You can watch both videos after the jump.