How do you incorporate a locational hit into your character's skills? Hitting a certain part of the NPC is dependent on the Player's skills, not the Character's skills. That is why it is not an RPG aspect. Unless we go Turn Based or using some derivation of a paused targeting system.
Otherwise, a Level 1 character with no real blade skills could pull off head shots if the player was skilled enough. Then we have the Lockpicking minigame problem, but in a bigger way.
Hitting the NPC is dependent on the player's skills, not the character's skills. That is why it is not an RPG aspect. Unless we go turn based or use some derivation of a paused targeting system.
Look at how Deus Ex handled accuracy when it came to characters with low skill levels. These are not difficult hurdles, they've been overcome before. There is no reason a similar system could not be employed with melee weapons, making you less accurate with a sword and more likely to miss the intended limb. And as has already been stated before, there is no need for one shot kills. The only difference would be a modifier to damage. Since low level characters aren't dealing much damage to begin with, a multiplier isn't going to significantly change the outcome of a battle.
Beyond that, we're already dealing with a game which employs a heavy degree of enemy level scaling. A head shot, even one that results in instant death, isn't going to make much difference when you're already capable of killing multiple enemies at once straight out of the gate. The only time I've bumped into an enemy I couldn't defeat, I was fighting monsters (trolls and giants). Damage modifiers would be different for creatures than for humans. Extra thick skulls could even result in damage
penalties to head shots, forcing you to target the torso or limbs. This adds further variety to the combat as you're forced to learn the strengths and weaknesses of the enemies you face. This works with humanoids as well when dealing with different armor types. If an enemy is wearing a steal helmet, aiming for their head isn't necessarily your ideal target.
I'd even be fine without any damage modifier at all. Just impose penalties on your opponent based on where they've been hit (similar to crippling a limb). Arm shots would lower weapon skills/damage, leg shots would slow or knock over an enemy, torso shots would stagger, head shots would stagger or reduce weapon damage or paralyze an enemy (stunning them). The chance of any successful effect is then based on skill level. Those who are less skilled will be less likely to induce these effects, those who are more skilled will induce these effects often.
The fact remains, these are all tactical decisions and will
always be based on player skill even if we
did move to a 100 percent turn based system. Excluding these options only results in a more monotonous combat experience. Instead of being able to overcome your physical limitations through mental ingenuity, you're forced to simply grind skills until you plow through anything in your way.