If nothing else PaRappa The Rapper broke all the rules. I remember like Chi, when PaRappa was released and some people dogged it because it was very simple looking game. It didn't have the latest in 3D graphics and state of the art Artificial Intelligence (AI).
PaRappa's concept based on rap, plays something like a cross between Simon-Says and Tekken-style 10-hit combos in a music video. While this may sound simple, it no where near demonstrates the depth of the game.