When I talk about RPG design I often refer to the Lord of the Rings. LOTR isn't my favourite fantasy series by any stretch but it sure is well known; everybody knows the world and the story to some extent and it is a useful benchmark for expectations. I don't like to put Tolkien on a pedestal, especially because of the racist overtones in the books, but his work is certainly iconic!
With that noted I would like to draw the reader's attention to healing. In LOTR Frodo needs to take a big break to heal up after he is injured by a Ringwraith. It takes time to accomplish this and that leads to a significant pause in the action. This is true of most other fantasy stories as well; at some point important people are injured and their recovery becomes a major plot point. Compare this to DnD or WOW though and you see the exact opposite situation. In nearly every case people recover from their injuries nearly instantly and move onward without a pause. Heck, in WOW it is normal for a main tank to go from completely healthy to very near dead every second or so for minutes at a time! So much for recuperation.
Now in WOW this makes sense. It would be ridiculous and unpleasant if characters had to take weeks, days, hours, or even minutes to recover from injury. It is a video game and sitting staring at the screen waiting for people to heal up is boring as hell. The core issue is that WOW is a real time combat game. You can't just fast forward 2 weeks and be at full health again like an author can with fictional characters. To keep the action moving and to make raiding and PvP possible characters need to heal rapidly.
Why is DnD in the same boat as WOW in terms of healing? I have been asking myself this and coming up pretty much dry on answers. In DnD you can fast forward time when it is dramatically necessary and there are all kinds of things to do that are not combat related. There is no need to have every character instantly heal to full every 5 minutes. The only stumbling block in DnD is that we wouldn't want characters to get injured and then be unable to fight for weeks on end; how would this work if the villain had to be stopped within 2 days and the a key party member got badly beat up? Leave them behind for the boss fight? Just let them die? There are no good options.
However, that lack of options is only true in the model of combat where characters have a health pool that in theory represents ducking, weaving, exertion and skill but which can only be replenished through healing magic. High level characters have a huge amount of HP which reflects their skill and determination but for some reason it takes absolutely forever to heal naturally; it is never explained why a 1st level fighter heals to full in 3 days and a 10th level fighter needs 40 days! What is needed is a HP model that reflects actual damage to the body and also exertion and exhaustion.
Back in the day my friends and I were building a system that had two HP pools. One pool called yah points (You're A Hero!) came back at the end of each encounter and regular HP returned much more slowly though it could be healed quickly through magic. I think we didn't go far enough though: We should have removed all magic healing entirely and let characters regain HP very slowly. Being injured and having to take time to recover is okay when you have yah points available since you can be careful in a fight and try to be defensive - you are at risk, but not going to be killed by any random AOE attack that lands. This allows characters to be injured and necessitates rest but lets people still fight a little when hurt.
It is annoying to have two HP pools to keep track of but I think it is worth the effort. I really like the idea of characters being injured without having them spending a lot of time in 'gonna die imminently' mode. The idea of taking downtime to heal injuries is great and I think I want to write a game where that is true.