I'd hazard that a good summary definition might be "game rules are universally applicable." In that if there is a rule/mechanism anywhere, it can be used every

I'd hazard that a good summary definition might be "game rules are universally applicable." In that if there is a rule/mechanism anywhere, it can be used everywhere, with anything responding to it. Or in Dwarf Fortress + OO/Smalltalk/message terms: every object can and must respond to every message. (to the extent technically possible)