In this Better Game Design through Data Mining feature, David Kennerly nicely describes how data mining can improve MMOG game design. He starts by describing the "why"

Why Mine Data? Because players lie. Player feedback alone provides a poor diagnosis of game design. The picture a player's verbal feedback paints is not even an approximate guide. It is a distorted portrait of psychological and social forces. Players do not accurately report their own behavior in surveys or customer feedback. (...) As political creatures, players, and developers, also revise their reports (...) Data mining also builds better theories. It gives the game designer insight into how players use and abuse the game. It broadens perspective, proves or disproves hypotheses, and substitutes facts in place of opinions

And then look at the possible statistics than can be used, taking as an example "performance" and how it can be derived from diverse indexes (experience point by time). The article also explores test and analyses of hypotheses that can give interesting insights to game designers. Finally, the author presents in which context this can be used, showing examples to balance the economy, catch cheaters, cut production costs or increase customer renewal.

Why do I blog this? I always have been interested in this sort of log mining of games as a complement for qualitative data (like interviews and observation; I am not a great fan of surveys). What is proposed here is simply speaking close to log analysis in human-computer interaction research. This methodology is additionally similar to what Play-On researchers are doing.

This idea also connects with what USC Justin Hall proposed with the concept of "passively mutliplayer games" (see also here).