Raph Koster, who has many critical insights on game design, has a great new essay on his blog called "Making games more cheaply," which closes with this statement that applies to practically every form of digital media extant, and may just be the secret to success in the 21st century:
Embrace prototyping. Make your game playable and fun before you have any art. Stop writing big design docs.
Big design docs are useless. There, I said it. Trying to build a game off of one is like trying to recreate a movie from the director's commentary track. They are largely castles in the air. The only time that big design docs serve a real purpose is when they are describing static content.
Embracing prototyping is a huge mental barrier for people. But it is what gets you to that long-lived self-refreshing systemic game design. You can prototype almost any game with some dice and some index cards. And plenty of ideas that sound good on paper turn out to suck when tried out for real.
Prototypes properly done are cheap. Prototyping is whistling five melodies and seeing which one you remember the next day.