I didn't like this rule in 4E, and the reason is that it bends the genre and creates permanent moral dilemmas.
It feels a little weird to see so many people insisting on making killing NPCs easier, even by accident. The usual complaint is that PCs are too ruthless, and kill too casually.
....
That's a good thing, if you ask me. In real life, true heroes don't do a lot of killing. Whenever I play anything but chaotic evil characters, I make a point of not killing when it's not strictly necessary (usually in self-defense or legitimate defense of others). While I love D&D, the amount of slaughtering of human-like creatures that goes into your typical adventure (my own adventures included) is really weird. Makes me think that players don't spend a lot of time thinking about the meaningfulness of the choices their characters make, and I think that should be a huge part of playing.
Thank you to the folks who have suggested that maybe it's actually nice to encourage players having their PCs incapacitate rather than kill....
Personally, I would jump for joy if my players' PCs knocked out and arrested every sentient humanoid opponent they encountered; it would strike me as far closer to an actual in-practice "Lawful Good" ethos than the chop-'em-to-bits standard of D&D, even heroic D&D.
In classic D&D, some monsters may be so awful that they are worth killing. Maybe just a few, maybe more. But beyond those others are so threatening that only real weapons and dangerous spells will stop them, in a way that tends to kill them, or at least leave them severally incapacitated for a long time. This is unfortunate, but thats how it goes (I have to be careful here, yes, there are real world analogies with this, and yes, there is a lot of controversy around those...but back to the game).
When you make it easy to knock out opponents with swords and shocking grasps, you have changed the genre. You are now playing a supers game, where only deviant heroes attack to kill. In this genre you are looking to apprehend villains, not kill them (though it if is a movie version, they may do a good job of getting themselves killed at the right time, but anyways). If you can knock out opponents, and just say to hell with it, and kill them, you are basically (mass) murderers.
Its the rules change that does this. It creates this dilemma if you think about it too much. (And if you are torturing after the capture, then its encouraging murder on the one hand, torture on the other).
Its true, D&D is bloody, maybe too much. An optional rule along these lines would be ok. But you would have to have conventions on when monsters just give up and won't trouble the players, and when you would need to capture them, and when it would be ok to kill them.
ALSO, if that wasn't a long enough post, it degrades tactics. Don't want to kill them, don't fight them in the first place. Want to take them prisoner, put in the extra effort. Use charm, illusion, abilities, a way to strike to subdue. But if you go at somebody with an axe, then no, they probably won't be fit to talk to afterward.
