Raph Koster is one of the smartest people I've ever met. And I only met him briefly at the Second Life party in S.F. during the Game Developer's Conference. I love his book, A Theory of Fun. And I recommend the book to everyone; parents, kids, instructional designers, software developers, everyone. Many of his blog posts go far deeper into game theory than I can understand, however this post is quite relevant to recent discussions and learning projects with Secondlife and other MMO tools.
As a mere lvl 21 Orc Warrior I'd say I'm at the second circle within WoW.