nobody can doubt that if there is an arrow that has pierced your skull, you are probably going to die soon. If hit in the chest, death will most likely come much later (in an exerpt from a norse Saga We had to read in my english class, a viking fought off a pack of wolves with an arrow sticking out of his "Fat Line" supposedly near his heart. he then died, but it was from blood loss.
You can't really say "it's POSSIBLE to survive that so it shouldn't be instantly lethal" though, most of those are "freak cases" more or less. There actually was a soldier who was shot clean through his heart and survived without immediate medical attention, in fact the injure was discovered years later and only because they found scar tissue on his heart.
The other way around some injures you'd think would be a 100% assure death can be survivable, there are people who literally lost half of their brain and survive it (albeit with very limited physical abilities afterwards) but you wouldn't really believe a NPC surviving a part of his skull being flung off.
The thing is locational damage and such should go by some factors, if you penetrated someones lungs it will cause damage and inhibit his abilities, if the injure is sufficient it will cause his lungs to collapse which is as good as dead or at least means "fight is over", note
SUFFICIENT damage. Same with a heart injure, there can be some minor injures that are survivable, enough damage and you're dead.
Similarly in game a brain injure can be survivable (in game it could be simulated as permanent skill damage) but enough damage and it's over.
One more reason for a LD system BTW is sneak attacks. You simply can not balance them in the classic system, they're either severely underpowered or ridiculously overpowered. With a LD system it would make sense why a sneak attack can be lethal, you can directly sting into someones heart, slit their throat so they bleed out, strangle them so they suffocate, snap their neck or cause a blow to the head that knocks them cold.
All those would be possible in regular combat but in sneaking you got one big advantage, surprise and giving you room to "aim". People don't see you coming, they can't defend or react in any way and while they don't see you coming you have time to prepare.
Limitations would also make sense, you can't stab someone in the chest when they wear daedric armor, but you can grab their head, pull it back and stab your dagger into their exposed neck, or snap their neck, or take a wiiiide swing with a heavy blunt item which, despite the helmet, could knock them cold.