Optimal Game Design

What separates an enduring game from a weak game? Why do some games last for 1000s of years, while others end up in the bargain bin after one month?

Necessary Elements Optimal Elements
Rules Easy to learn, a lifetime to master.
Interaction (participation) Players enter into the created world and forget about the real world.
Opposition Balanced. Skill, not luck, is required to win.
Decision Making (non-linear) Interesting decisions. All options have merit.
Goal The goal, and the players' relationship to it, is clear. Dramatic tension increases as the game reaches its conclusion.

If an activity contains all the necessary elements listed above then it can be considered a game. But when you design a game don't aim for necessary. Instead, set your design goals higher and try to achieve optimal results.