Craig Morrison's latest personal blog entry might be right up your alley. The Funcom executive and current Age of Conan boss has posted a lengthy look at his thoughts on massive design, and more specifically, progression.
Morrison examines progression versus entertainment, and goes on to contrast the need for the former with the desire for storytelling. He also touches on the vast array of player motivations, ultimately concluding that while "progression, and the need for it, is a bedrock of the expected player experience," future MMO designers should be mindful of the differences between what they want the player to do and what the player wants to do.
Clearly, there is no easy answer to the question of how does one design a good MMORPG. After all, one gamer's soul-sucking grind of death and despair is another's relaxing evening at home. That said, Morrison's perspective illuminates at least one line of developer thinking and is worthy of a read-through whether you're a dev, a player, or both.