Game Design Wiki: Maximize Fun Minimize Frustration

In Game_Design:_SotS?, Bruce_Shelly? sums up his game design tips with this rule: "Work towards two goals: maximum player fun (gameplay) and minimum player frustration (random events, [1] poor interface, bugs)."

This is an extremely high level and general principal - it probably won't help you design specific rules, but it may help focus areas to grow or cut back.