From the live chat today...
ExtendedRest: Is there a plan to deal with long term wounds? Right now having all health and everything reset after a long rest seems a little too easy. Especially with as little healing options as a Next party have access to on their own right now.
Jeremy Crawford: We're not likely to make long-term wounds a part of the core, but we have discussed providing a wound option for DMs to incorporate into their campaigns.
Mearls: We erred on the side of letting long rests heal everything, primarily because we were fairly split on how to treat it. Personally, I'd like to see a rule where you get back a certain amount of hit dice each extended rest. It might be based on Con and/or class. I have to admit that the current rule picks at my sense of realism.
To follow-up what Jeremy said, I've toyed with a wound system where you get some effect each time you drop below 0 hp, to represent a bad injury, For instance, broken bones, strained joints, concussions, etc. But that would be a rules module.
Jeremy Crawford: This is another example (the long rest) of us leading with the powerful version of something with the expectation that we might end up dialing it back, based on playtest feedback.
I'm a bit confused by the bolded piece, and by the calls for "realism" in healing in the playtest. First off, read the description of hit points in the playtest packet. Bottom right column, page 12. Hit points are NOT purely a physical damage meter. According to the description, in fact, you only take actual physical damage when you drop below half max hit points, in the form of minor cuts and bruises. Only when you drop below 0 does your hit point total indicate any actual harm to the character.
Let's be up front though. Hit points have never be a realistic representation of anything. Fiddling with their rate of recovery won't add or subtract "realism" to the game. Completely scrapping the subsystem and designing something else, with realism in mind is the only way hit points will ever be associated with "realism." To make matters worse, Mearls is the head of the show on this edition, and yet he doesn't seem to get his own edition's description of hit points. You have to have at least 1 hit point left to recover naturally with a rest. Great, because even at 1 hit point left, you're still only bruised and have a few minor cuts. It doesn't get real until you hit 0 hit points.
Hit points as written in the playtest are a mixed bag health bar, where you're only really hurt when you drop to 0. How is a good night's rest allowing you to recover from a few cuts and bruises not realistic? Note the rules, you can't recovery with rests of either variety unless you have at least 1 hit point left, which according to the packet only represents cuts and bruises. Real trauma is only inflicted at 0 or below. Which is where magical healing comes in.
D&D has never been a "realistic" game. Arguing about realism over something as unrealistic as hit points is more than a bit silly. This is a fantasy game with wizards, gnomes, gnolls, ogres, dragons, ancient gods, divine magic, paladins, and liches after all. But somehow recovering from minor cuts and bruises overnight shatters realism? Come on.
"And why the simple mechanics? Two reasons: First, complex mechanics invariably channel and limit the imagination; second, my neurons have better things to do than calculate numbers and refer to charts all evening." -Over the Edge