ACM Queue has an article exploring the challenges of developing a reliable platform for an MMORPG, specifically looking at Wish by Mutable Realms. It discusses the design requirements for building such massive games. From the article:
"A common scalability problem for distributed multiplayer games relates to managing distributed sets of objects... A player may not be a member of more than one guild, or a guild may have at most one level-5 mage (magician). In computing terms, implementing such behavior boils down to performing membership tests on sets of distributed objects."
Click here for the full article.