Nine Hands
Explorer
I like all the healing stuff in 4E. What I would like, that isn't in it, is if you drop to 0 HP or below, you incure a penalty.
This would be a permanent injury. It can be healed eventually, but not with a short or extended rest.
The permanent injury would be treated like a disease. It could get worse or better. The worst bit of the injury - if untreated - would be something along the line of Gangreen(?). You must cut off the arm or die.
Or you loose the eye - now you have -2 to -4 penalty to all perception checks.
This should make the PCs desperate to not fall to a dying state and if they are careless and do they will want to take care of the injury.
Of course higher healing will take care of allot of these things - but that should taken care of in the system - it needs to be a risk even at higher levels. Perhaps these things can only be healed (magically) with a complex ritual.
While I think that this is an interesting way to go, I don't agree that 0 HP is a good point to start doing this. Instead, how about allowing characters to ignore a result of Death (due to three failed Death Saves or losing too many hit points) and instead suffer the penalty. This makes it a viable solution for character death and does not happen every game session or so.
A Song of Ice and Fire uses a similar mechanic, where you accept an ability score penalty (forever) to keep on living, what that penalty applies in character is up to the player. So a facial scar might be a penalty to Charisma, a penalty to Dexterity might be a maimed arm, etc.