Mark Venturelli has posted an interesting article about principles of game design structure:
Designing a game is designing a space of possibility. It is the creation of a structure that will play out in complex and unpredictable ways, a space of possible action that players explore as they take part in the game [Zimmerman and Salen 2004]. In a game of soccer, it covers all the possible movements of every player for every individual position of the ball in the field, every foul and every goal, and so on. It is the collection of all possible actions and outcomes inside the designed space of the game – all actions and outcomes artificially made possible by the system. The first goal of this work is to investigate how some techniques of restriction of this space can contribute to make fun casual games that can stand on their own in long-term play.