I think prototyping with placeholder art works for most games, but not all of them. Prototyping gameplay is important for games where the gameplay is king and

I think prototyping with placeholder art works for most games, but not all of them. Prototyping gameplay is important for games where the gameplay is king and art/story/setting is something on top. But if your game is going to be a regular platformer and the cool thing is the art, or if you're making an exploration game, etc, you should probably start with mockups and then make the game.