Five Ws of Game Design
During the course of trying to improve my game design chops, I came up with these five Ws in order to help guide my thoughts:
Who is the intended audience?
Think about who will be playing your game. Games are a product, and products need an audience. Some of the other Ws will influence this answer, specifically "What", "When", and "Where".
What is the core experience?
Think about the core experience you want to deliver. How should your game make players feel?
When will the game be played?
Think about the times when players will play your game. Is it a daily habit akin to crosswords or Scrabble? Is it something to play on the weekends or evenings with or without friends?
Where will the game be played?
Think about the places where your game is meant to be played. It could be a mobile focused game meant to be played on the bus. Maybe the player is sitting at home on the couch, or nestled into their favorite chair armed with their mouse and keyboard.
Why is the game unique and interesting?
Think about the reasons why players would want to spend their time playing your game.