I've noticed quite a few versions of the "rules" in this thread. People want to make up their own to suit their idea of common sense--and all of the versions people have posted make sense. The problem comes when people with different ideas of the rules meet. Life is easier if everyone agrees on the rules, however arbitary they are. Otherwise we're like the self driving car at the 4 way stop that gets stuck because it can't figure out whose turn it is. That doesn't mean the rules have to be followed blindly when it's fairly obvious what to do--if I'm coming down and a heavily laden backpacker struggling up steps aside before I have a chance to, glad to take a break, I'm not going to step aside myself and glare at them for breaking the rules.
As far as horses, they get the right of way because they're huge, potentially dangerous, and dumber than most people. BTW if you get out of the way of a horse--get uphill. Better kicked in the shin than the head.
Mountain bikers--the more you ride with consideration for hikers the fewer the trails you'll be banned from.
Runners--I'll get out of your way when you're coming down and I'm going up but you don't have any god-given right to have ROW over everyone else on the trail.