In 4e, there is a big focus that HP includes morale and other factors aside from physical stamina; but with the bloodied condition, and the flavor text of abilities, some people have problems really closing that gap. Which is fine; we are kind of trained to think about HP as a health bar more than as a "total sum ability to fight," especially if you play video games, and especially if you are used to systems such as 3e and Whitewolf where supernatural healing is the only healing available.
One part of the gap is wanting to have a clear and distinct notification of when you are actually physically taking damage. Some say that you start really getting hit when you're bloodied, but that doesn't work so well for some people, as that can get healed back up with an "attaboy" or a breather. More people want there to be a real reason to be afraid when you start getting hit; and while a Wounds/Vitality system doesn't really work for the genre that D&D tends to work with, this is an okay idea, because it pushes at some realism.
While working on my side project, I realized that there is one "layer" of HP that is definitely tied to your ability to take hits: the + Con Score that you get at character generation.
Imagine, if you will, that everything BUT that con score is your ability to dodge, deflect, suck it up, and absorb; that only that last 8-30 HP is your actual body. This defines clearly when you start taking full damage, because it's actually tied to an ability score.
Now, I want to think about taking this one step further; imagine if instead of adding the con score to your HP, instead you had an additional attribute called Vitality; when you were out of HP, you took damage to your vitality.
Now, imagine if this HP could only be recovered by magic, the heal skill, or by resting, with the rest restoring your con modifier to Vitality. Your HP is your "narrative" punishment meter, while vitality is your "realistic" one.
So long as you had any HP, even if your vitality was at 0, you could act; you would literally be pushing yourself beyond your limits, which is the flavor description we like to use for the Morale based HP restoration method found in 4e. However, you would have a lower maximum HP, making repetitive injuries more dangerous to you.
You could even, if you are so inclined as to want to put a penalty, give them a -1 to attacks, damage, skills, and defenses, while they are at 0 vitality.




