Saying the rules are the physics of a game world implies a greater connection than it really is. Taking a varying amount of dammage when you fall from ruleset to ruleset is nearly the same thing as not taking any dammage at all, or not being able to fall at all because there is no gravity.
When you said that dropping a sword and it always falling to ground is an implied rule, what you really talking about is the physics of that world. I can play D&D and have your implied rule not be true. Heck, WotC's even put out a book of such worlds, Manual of the Planes. To me, the physics of the world is a feature of the setting, not the rules of the game. For one thing, rules encompass things physics has nothing to do with, like diplomacy checks against NPCs.
I imagine that many of your implied rules would be features of the setting to me.
