DAMNLAG - Modern Warfare 2 and the Art of Manipulation. Thanks Mike Martinez via Digg.
Game makers have known for years that the simple act of leveling up can drive addictions or at the very least make people play more. In any number of RPGs leveling up is crucial to advancement and developers have noticed that people will actively fight the concept of playing to grind their way to a higher level by doing boring repetitive tasks. For reasons unclear Infinity Ward was the first developer to apply a level system to an online FPS with the tried-and-true RPG model of greater perks and abilities. Before this, FPSs were a genre unfamiliar with lending precise order to such things. Sure enough leveling became key to the Modern Warfare experience.

Bitmob - Indie Games Should Keep Acting Like Indie Games.
Not many require that all painting be aesthetically pleasing. Likewise, not many claim that all video recorders should only produce entertainment. We don't hear arguments issuing that any other media must serve one, baseline purpose before they can do more. Such assertions would be arbitrarily reductive. Francis Bacon's work isn't fun or pretty, but it is interesting and valuable. Similarly, the works of Stan Brakhage and Maya Deren aren't entertaining, but that doesn't imply pointlessness.