http://www.vancouver.wsu.edu/fac/peabody/game-book/Chapter5.html

A game must have a clearly defined goal. This goal must be expressed in terms of the effect that it will have on the player. It is not enough to declare that a game will be enjoyable, fun, exciting, or good; the goal must establish the fantasies that the game will support and the types of emotions it will engender in its audience. Since many games are in some way educational, the goal should in such cases establish what the player will learn. It is entirely appropriate for the game designer to ask how the game will edify its audience.


http://www.gamedev.net/columns/books/bookdetails.asp?productid=384