If you are dishing out additional penalties on top of the extra damage, you are either a sadist or you don't understand math.
I must be a sadist! In my RPG, you roll on a d% Hit Chart
every time you damage someone's HP. It goes against enemies, and it goes against players. Fun will be had by all!
This is what I was saying. Sure, Lingering Hits sound all realistic and gritty but getting the crap beaten out of you every single fight is not fun for players.
You're right (for most groups, I assume). Maybe that's why my players avoid getting the crap beaten out of them now
It also means you can no longer run a party without a healer (it was merely a good idea before, now it would be mandatory).
This wasn't true in my RPG, and I doubt it would be true in 5e. I've run healer-less campaigns for many 3.X parties (years ago, when I still played the game), and have done so for my fantasy RPG. It changes the dynamic, no doubt, but it's nowhere even approaching impossible. While I haven't played 5e myself, I don't see what the big difference would be (especially with some self-healing available via hit dice).
I would probably give the PC the option of taking a crit or taking a lingering hit instead, rather than making it mandatory.
This is a good approach, too. I also like the idea of it applying to "maybe you didn't die after all" for groups that want to avoid a character death or TPK. If you take enough damage and you'd normally die, maybe you can roll on a permanent injury chart instead (lost limbs, etc.). It sucks, sure, but here's hoping you get a Regeneration spell cast on you! It's not too different from hoping someone casts a Raise Dead on you, except you get to play in the meantime.
Oh, and feel of the game, of course. It wouldn't fit every group (and probably not one of my groups), but I thought I'd throw it out there.