Variable Rewards

Schell says that "the power of variable rewards over fixed ones has been proven in thousands of psychological experiences" (though no experiments were referenced). The idea that it is more interesting for a player to be occasionally given a reward than always given a reward. For example, when you capture a Pokemon, you are sometimes given extra items and even more rarely they change into another Pokemon after catching them. The surprise stays with you a longer time, even if the frequency of rewards is the same.

Related to the idea of Triangularity In Game Design and the taking risks component of David Perry's Key to Addictive Game Design.