Koster’s post on this topic explains the importance of balanced economics in commercial virtual worlds:
From time to time I think about this in terms of the design of educational virtual worlds. Economics should be equally important within these worlds and it seems that this is often neglected in their design. I believe that status can be a sort of “currency”, i.e. points, awards, badges, roles, etc., especially if these are restricted and shared among players/users. I have been playing around with these ideas for the design of our Virtual Learning Landscape, and with the virtual Fish Tank idea that I discussed at the recent Game Based Learning summer school I think that tradeable items (or sendable) in fish tanks that are representative of learners could improve the long term engagement with the virtual world.