Mark_LeBlanc? at http://world.std.com/~mahk/algorithmancy/ discusses this.
Jean-Paul_LeBreton? also - http://www.antifactory.org/archives/000021.html.
Fun is a general enough word to be almost meaningless. (A contrary point of view is expressed at Fun.)
Washing dishes is fun, if you're in a dishwashing competition and you have to wash more dishes than your foe. Games have to have more and better and different kinds of fun.
For example, most action-adventure videogames are trying to do much more than be fun. They have to create a new virtual experience that the player wants to have. I may be in the minority here, but I often find mini-games and sub-quests annoying. Thank God you didn't actually have to complete the fishing challenge in Zelda, for example. I don't play Zelda so I can fish. (This ties in with SimpleAsPossible.)
In other words, a game can be 'fun' and not necessarily achieve its true goals.
And it's not one of the words you should use even if "you have no words and must design."