Yeah, I like the idea of injuries more narratively than mechanically. I love the idea that you *could* lose a hand and then have to replace it with a mechanical or magical one. I love the idea that you *could* end up with a character that has like, the Batman scars of a life lived in combat. And I like the idea that those can happen as emergent from the game itself in specific fights.
I definitely do not like the idea of it being tied to a set amount of damage taken. It has to be some sort of rarely occurring mechanical add on that has some sort of prerequisite. I almost think a critical with a confirmation roll, and then a roll on a chart to define the injury would work, but then it might be too infrequent. Like I say, hard to find the right balance. It could also be something tied to bosses specifically, and that *might* work for how it usually happens in heroic fantasy, and it would certainly make bosses more scary, but I have a feeling it would adversely affect the psychology of players entering into a boss battle (making timid players even more anxious, encouraging over-planning or cheese tactics always, etc)