MMO developers and publishers try to provide game-play that keeps you coming back again and again so you won't stop their money flow by canceling your subscription. The easiest way for them to do this is to make their games an addictive grind. You feel compelled to level up. You can't help yourself.
Just because something's addictive, though, doesn't mean it's fun. But is there another way? Damion Schubert (of Meridian 59 and Ultima Online fame, and one of a certain blogger's personal heroes) posed that question on his Zen of Design blog.
MMOs must be centered around highly repeatable activities, Schubert said. Combat, for example, works well because developers can put in a lot of changeable variables to make the experience different every time. On the other hand, he uses puzzle/mystery games like Myst as examples of games not based on a repeatable activity. Once a puzzle is solved, it's solved, and that's the end of it. So, if not that, then what? What other games have mechanics that can be used as a model for MMO game-play that sticks?
That discussion is going on right now at Zen of Design.