You're designing a game. Where do you start? If you said “gameplay” you've just warmed my designy-heart, but you still get a hearty, “not so fast, buckaroo”. Even if your game is pure gameplay, and abstracted to Timbuktu and back like Tetris, Trism or Bejewelled, you still need to have a guiding principle. You want to have something that you're looking to capture, a feel, a rush a challenge. If you don't work that out early on, you tend to get a weird Frankengame where the freaky-big arm punches the opposite leg as it walks.