Good question. Here's a very interesting article about fun from Colin Johanson, lead content designer for some other MMO. It's all about, fun. The paragraphs that hit me in the eye:
"The answer can be found in the mechanics and choices made in subscription-based MMOs, which keep customers actively playing by chasing something in the game through processes that take as long as possible. In other words, designers of traditional MMOs create content systems that take more time to keep people playing longer. If this is your business motivation and model so you keep getting paid, it makes sense and is an incredibly smart thing to do, and you need to support it.
When your game systems are designed to achieve the prime motivation of a subscription-based MMO, you run the risk of sacrificing quality to get as much content in as possible to fill that time. You get leveling systems that take insane amounts of grind to gain a level, loot drop systems that require doing a dungeon with a tiny chance the item you want can drop at the end, raid systems that need huge numbers of people online simultaneously to organize and play, thousands of wash/repeat item-collection or kill-mob quests or dailies with flavor text support, the best stat gear requiring crazy amounts of time to earn, etc.
But what if your business model isn’t based on a subscription? What if your content-design motivations aren’t driven by the need to create mechanics that keep people playing as long as possible? When looking at content design for Guild Wars 2, we’ve tried to ask the question: What if the development of the game was based on…wait for it…fun?"