What is expression?  In game actions as expressions of player choice. 

Example, Tetris:  Clearing as you go or holding out for the 4 line combos...  Tactical expression.  Forza:  Decal custom cars, as visual expression, no impact on gameplay.  Tuner Kits, some tactical choices, but not required.

Game reaction to cosmetic choices is limited (rpg/bunny suit, no one notices).
RPGs equipment attached to gameplay, but cosmetic expression is limited because the player tends to take the tactical choice.

Dialogue:  Monkey Island, KotOR.  Mostly cosmetic, you can always go back and choice the options.  Some real impact, ie Mass Effect / BioWare games.  Style/Tone carried through. 

Alpha Centauri, faction based RTS, has “style” expression based play.

If it’s not a choice it’s not a form of expression.

Music games - play is mechanical.  Expression (dancing/rocking out) is external.

MMOs - chat behavior / sharing a path / gossip / smack-talk.  Mastery of mechanics.  “Looser, play some more.  I’ve played more than you.  You suck”.

GTA/Open World. 

Fable/Black & White.  Direct self expression.

Purchase decisions / achievement points. blogs / youtube / community / mod building / etc.

In the game vs around the game.  Do actions matter in game. 

Mastery is not expression.  (Mastery is not bad, but it is not expression).

Add expression: More closure/in game response.  Less punishment, softer fail states.  Encourages exploration.  Monitoring player actions and trigger softer environmental changes.  More procedural content, or hire more content creators.  Replace length with breadth. 

Indie examples, see:  Chivalry is Not Dead.  Façade.

Wii & Online.

Pure sandbox often lacks the “events” that acknowledge the players actions. 

Expression allows players to do what the want to do, not what the need to do.

Why bother?  Provides experiences worth careful choice, and talking about.