Anyone interested in this should check out Jenova Chen's thesis on Flow (which is the application of Flow Theory on games).

http://www.jenovachen.com/flowingames/foundation.htm

http://en.wikipedia.org/wiki/Flow_(psychology)

In particular the three rules that Chen lays out:

   1.  As a premise, the game is intrinsically rewarding, and the player is up to play the game.
   2. The game offers right amount of challenges to match with the player’s ability, which allows him/her to delve deeply into the game.
   3. The player needs to feel a sense of personal control over the game activity.

Raph Koster also focuses on the same issues in his very accessible book A Theory of Fun.

Weirdly enough, I didn't much find flow fun... It seemed very flat or boring. And linear. I think the best part of the game was the atmosphere the art and sound created, not so much the gameplay.

And that's not to say that his theory isn't interesting - it is - it's that I think it's very hard to define "fun", just like it's hard to define enjoyable music.