What it is
Player expression is the depth of a game’s systems that lets each player choose how to play: how to fight, how to move across the map, how to look, how to make money. A game with high player expression feels like yours, because two people can play it very differently and both be right.
How it shows up for me
It’s the recurring theme in the games I love, and the thing I’m chasing in the game I’m building. My test of depth: I’d used maybe sixty percent of one game’s combat system and was still having a great time. If a system is that fun at sixty percent, it’s probably very well designed at one hundred.