Game Perceptions #1 - HP vs AC

This is going to be a series about how things are perceived in the game and possible suggestions to make those perceptions easier to deal with. Also, this post and the next few are being written while I am on Vicodin for pain. So if I am more rambley or hard to follow that usual, please forgive me. I hurt.

So Armor Class (AC) is a measure of how difficult you are to hit and Hit Points (HP) is a measure of how much damage you can take. This seems fairly straight forward. If I roll well enough to hit you with my sword, I get to do my damage to you. Things begin to break down when you factor in levels.

If I swing a sword at a commoner, I'll probably hit. I'm also likely to do enough damage to kill them. However, if I swing the same sword at high level adventurer (assuming I hit), I'll do the same amount of damage, but it won't kill them. They're still human, so the sword should be as lethal to them as to a commoner, but clearly it isn't. They can take more damage, but how is that represented?

I think a good way of looking at it is that AC is passive/reflexive defense, you don't have to think about it your training, experience, and plate armor allow you to shrug off blows, while HP is active/intentional, you need to devote some attention to the blow that actually landed. So just because a high level adventurer got hit by a sword, it doesn't mean that they were actually injured. An intentional twist avoid the blow, grunting as your shield absorbs the impact, a frenzied parry; all of these things can represent HP damage.

So when do you actually start getting hit? There is no hard and fast rule for this. I think that if you are under 20 hit points or so, the hits you take should reflect some damage. Whatever is the most dramatic. That being said, critical hits should always do damage to the person. The players should be rewarded on a critical hits with drama and action.