ON topic: The great and mighty Mearls solves nothing. When you use a harpoon with a rope attached, you need to know if the damage you caused was due to hitting the person because you can yank them towards you in your next action. For injury poison to do damage, it has to cause an injury. If you're making a save vs. poison, it can't be paired with sword damage that's described as caused by muscle fatigue.
A harpoon can snag on armor or clothing. A sword delivering poison need only inflict a scratch, and fictional heroes "shake off" the effects of things like poison all the time. These are easy explanations to handle with an abstract HP system.
And if you've been bathed in acid, bull-rushed into lava, sliced with a poisoned knife, thrown off a cliff, and harpooned, you're not going to be better after a few hours of rest. You're not tired and bruised; you're burnt, poisoned, stabbed, exfoliated, and have broken bones.
Total immersion in lava or acid? Well, if the character doesn't take enough damage to go unconscious or even warrant a healing surge (spent surges being a better indicator of the character's actual state of health, as others mentioned), obviously his clothing/armor protected him from the worst of it, or he was able to minimize contact with the substance through reflexes, luck and/or ingenuity.
If the damage is more serious, then sure, the character has burns and serious injuries. And, just like most Hollywood action heroes, after resting a bit, he'll be ready to keep going.
There needs to be a plausible narrative explanation for why you're better. Spells are that widget. A nap is not that widget.
The thing is, most parties
did have a healbot, and didn't spend days healing up. 4E recognizes that pattern from previous editions, and abstracted it away, so that parties aren't
required to have a cleric to avoid spending days healing up before they can continue the adventure. This is a good thing, IMO.
The real issue is where do you draw the line for your own personal suspension of disbelief, because no D&D edition
ever modeled the types of injuries you describe. Your line seems to be recovery time (even if it rarely comes into play due to magical healing.) But was that even realistic? Why did fighters take longer to heal than wizards or rogues, for example? Why did a character reduced to 1 hp always take exactly the same amount of time to heal regardless of the nature of his injury? And if all that damage did represent being burnt, poisoned, stabbed, exfoliated and having broken bones, why were characters perfectly fine all the way to 1 hp, despite having been swimming in lava, then suddenly go to unconscious and dieing from a minor knife wound? Where was the character hobbling on his broken leg, or handicapped by the intense pain of severe skin burns? Not there.
HP's have always had limitations representing what "really" happened to a character.