The problem with 5e healing is that HD are terrible replacements for healing surges.
For those not in the know, in 4e edition, in addition to their actual HP, PCs had a number of ‘Healing Surges’ that would fuel healing and inform how much a certain effect would help you recover. Each Healing Surge had a value equal to a ¼ of your max HP. The Cleric sends a Healing Word your way? You can spend a Healing Surge and you regain its value in HP and a bonus. Take a potion? It lets you spend a healing surge plus a small bonus. Better potion? TWO Healing Surges. Take a rest? Spend Healing Surges to recover their value in HP.
This had a number of advantages:
It meant that healing wasn’t just spending the healer’s resources, it also spent resources from the person receiving that healing. It, in turn, put a harder limit on the amount of healing that could be spent out during the day. You could have a full team of classes capable of healing and a Handy Haversack of healing potion and you would still not be able to go on forever. This had the bonus that strong healing effects just depleted those ressources fasters, being a trade off.
It’s a system that could easily have been adapted to make the game deadlier without making days shorter. Say you have a character with 20 HP and 6 HS, each worth 5 HP. That means that in the course of a day the PC in question can take 50 points of damage, BUT it means that a monsters only need to inflict 20 HP worth of damage to bring that character down. Additionally, PC would sometimes not spend a surge if it meant ‘overhealing’ and wasting part of it. You’d end up in a lot of situations with PCs not fully healed, so that danger could be greater. By playing around with the max HP and number of surges of PCs, you could dial a certain level of danger of your choosing.
And finally, it gave you a resource you could take away and have an actual impact for non-combat situation. Certain strenuous activities could cost healing surges, certain dangerous environment and traps could just skip the HP damage and just cost you a healing surge (basically, assumes you took damage and then spent a surge to avoid it). It basically gave you a measure of your PC’s exhaustion.
Heck, they could have made rituals cost healing surges, making them less ‘free’.
Healing Surges were brilliant and HD are an anemic attempt at the same mechanic with a legacy name, but they don’t actually do everything Surges could do and their potency is pathetic as a result. Plus you have to ROLL to use them? Bleh.