Mike Mearls mentioned that he was working on the Wounds / Vitality system for 5e based on Wounds reducing your Max HP, like energy drain does. He mentioned that it was set up so a portion of damage went to reducing Max HP, so if you have 10 HP and are hit with 8 damage, your current HP is 2 but your MAX HP is say, 6.
My question is how to divide this up? You probably don't want Crits to all go straight to reducing Max HP, and you probably don't want to have to track a reduction in HP each time you're hit.
What if you reduce Max HP if the damage is greater than your CON score?
I do this currently with my 4E game, where in addition to the Bloodied condition at 50% HP... I also have Bruised at 75% HP and Injured at 25% HP.
Whenever a player is dropped to 0 HP... they are knocked unconscious and also get a wound-- which gives them the Bruised condition. They can still regain HP as normal after the fight via healing surges and whatnot... but their Max HP is now at 75%. So if they normally have 40 HP... they can only heal back up to 30 HP now. If they were to get knocked unconscious again in a later fight, they move down to the Bloodied condition and their Max HP is only 50% of normal (by this example 20 HP). Third time goes to Injured (and 25% Max, or 10 HP), fourth time is Dead.
In addition, I also combine this with the Death Saves mechanic... when a failed Death save also drops you a wound level. So if you are uninjured before a fight (at normal Max HP) and get knocked unconscious... you drop a wound level to Bruised. Now because you are below 0 HP (knocked out) you have to make Death saves. You fail the first save, your wound level drops to Bloodied. Fail the second save, you drop to Injured. Third failed save you die (both because of the normal rule that says three failed Deaths saves kills you, but also because you lost that final wound level.)
These Wound levels cannot be regained through normal healing methods that require the spending of healing surges (like all the Healing Word type powers, potions of healing or the like.) Anything that requires the spending of healing surges does not recover wound levels, only your HP as normal, up ot whatever your wound level's Max HP is now set at. Wound levels are only removed through bed rest for a duration as determined by the DM (and I make that call usually based upon how many fights the PC was in, what they fought, where the party is resting etc.)
I also probably would have allowed wound levels to be regained via powers that allowed healing of HP without needing to spend healing surges (like the Cure Light Wounds power for example)... but as none of my players have any powers like that (all healing powers they have all require the spending of healing surges), I do not know if that rule would have made wound levels trivial or not.
I look forward to seeing how wounds / long term healing / HP/VP / reduced max HP etc. are all offered up as variants in the new edition and in the DMG. It will be interesting to see what they come up with.