The setting.
My game tries to be "meta" in messing with the medium and in doing so, requires the player to understand who they are and where they are for the effect to work.
I really couldn't decide whether I wanted the setting to be dystopian, futuristic, WWII, modern day, etc. The choice I ended up with definitely played a big part in the final game and what kinds of psychological manipulation I can impose on the player :~)
It's hard to explain, but here's a little teaser from it:
Which button would you press?