I'd like that sometimes critical hits mean a "permanent" effect, like a scar, a missing finger or eye, a leg injury and the like. Some of this effects could be heal with powerful magic o healing skils,
On the other hand the most severe may be permanent, but it's consequences may halved with the appropiate means.Other ways to make not all severe injuries permanent would be healing them with a mission (to recover a very strange and rare herb that grows in a certain place, or finding a long lost legendary healer,...), or using some items that are limited (meaning that you have to decide using it in one injury or in another). This could also affect to NPC members of the party, and the main character could heal them easier.
That could mean that a NPC of the party must retire eventually due to his grave wounds, but since there are more NPC who may substitute him it doesnt mean that the rest game is affected. This would encourage that more NPC are used during the game and make each encounter more intense.