I use a "colorful critical hit" system imc... in brief: when you crit, the damage vs. CURRENT hp of the target determines the severity; then you roll location. From there you roll exact effects (it's actually quite simple, the whole system takes only one handwritten page which I have paperclipped inside my screen).
So you only cut someone's head off or pierce their heart if you kill them.
The lowest severity effects (less than 1/2 current hp) include things like knocked back 5', knocked down, etc. A broken bone is usually over 1/2 hp but still above 0, whereas limb loss (yay! a use for the regenerate spell!) would occur when you get knocked below 0 but don't die.