The construction of a new roleplaying game must of necessity answer the question "What happens when you get punched in the face?" The answer to that question is of critical importance to game design both in terms of flavour and numbers. If getting punched in the face leaves you on the floor gasping for air and takes days to recover from the players are going to be cautious and careful of getting into violent encounters - much like real life! If getting punched in the face is trivial and getting fixed right up is easy then brawling can be a constant source of challenge and entertainment.
This week Mike Mearls tried to answer this question for DnD Next in his post on Hit Points. He talks a good talk a points bringing up how HP has always been viewed as a combination of physical damage, exhaustion, and psychological trauma but kind of goes off the rails when he talks about actual execution. As usual the team seems to have a lot of good ideas but is hamstrung by history and the expectation that they will produce a product that shares the mistakes and lemons of the past. I feel their pain in that they aren't allowed to make the best thing and have to compromise but I think that they innovate too little and compromise too much.
In terms of actual problems I think the king is the idea that players who rest in dungeon / wilderness settings can naturally heal themselves up to half HP. In order to restore their HP to full they will have to venture back to town to rest in a secure setting. I can't imagine how this could match up with any normal lore - the first blows you take are savage physical damage and after that you can just heal up any subsequent trauma with a little rest? What? In video games towns are magical places that can restore you to max HP instantly but how exactly does that make sense in a tabletop RPG? Why are characters who spend a lot of time travelling always half dead? Why do healers say "No, I won't heal you now. Wait until morning when you are only half dead and I will heal you then."
The system needs some kind of divide between HP that return rapidly and which represent exhaustion, effort, and light battering and HP that represent actual wounds and trauma. Having this drastically reduces or eliminates the need for healer classes (which have always been a problem) and allows the system to reflect the way stories and movies deal with getting hurt. The DnD Next team keeps dancing around this solution creating ever more silly and strange mechanics to address the problem that flat out don't work. Just accept the inevitable folks; you need a split HP pool. Or, you know, don't accept the inevitable and give Heroes By Trade a greater leg up on you than it already has.