Re: It requires a lot of feats.
Yes. Yes it does. On the other hand, it shouldn't be something that everyone is good at. Nonlethal damage isn't just your ability to knock someone out -- it's your ability to knock someone out without them suffering any long-term damage whatsoever. That takes training. Most "knockouts" are really just melee damage that goes over the threshold and drops the person to -1, at which point the person makes a Fort save to stabilize...
Take a good look at two different mechanics for me.
d20 Modern: One hit to knock out
D&D: Many hits to knock out
Conclusion: d20 Modern allows for the one-punch knockout.
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d20 Modern: Possible to knock out anyone with a single hit.
D&D: Without rogues, impossible to knock out anyone level 5 or higher, since you just can't do that much subdual damage in one attack.
Conclusion: d20 Modern allows for more powerful people to be KO'd, making it possible for the heroes to karate-chop the bad guy on the back of the head before he can raise an alarm -- whereas, in D&D, if that bad guy had 50 hit points, he'd certainly have a round or two to scream bloody murder even if James Bond himself were doing the chopping.
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d20 Modern: Damage is calculated hit by hit.
D&D: Damage is calculated by a separate hit point total.
Conclusion: d20 Modern allows for slightly faster play, because there are fewer numbers to track in terms of knockout damage.
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d20 Modern: A character can be knocked out with nonlethal damage, which has NO chance of long-term damage or death, or by a single lethal blow that exceeds his Con score, followed by a failed Fort save, which takes the character to -1. Once at -1, he gets a Fort save each round (DC20) to stabilize.
D&D: A character cannot be knocked out except through subdual damage -- the massive damage threshold is 50, not Con-based, and a failed save causes death, not dying-ness. A character who is down and dying has a flat 10% chance to stabilize, rather than a Fort save.
Conclusions (this one is important):
1) In D&D, going into negative hit points is extremely dangerous -- you never become more likely to stabilize, and unless someone heals you, there's a good chance that you'll die. There's also no mechanic by which otherwise powerful people can be taken out with one well-executed hit by any character except possibly a rogue with a lot of sneak-attack ability. Rangers, the deadly snipers of the woods, cannot knock people out. Fighters, for all their martial training, cannot knock people out. Monks can do it eventually, but not with one hit. Only a rogue, and only against a relatively weak opponent.
2) In d20 Modern, on the other hand, going into the negatives is not that big a deal in terms of immediate likelihood of death. Your character gets better at making Fort saves, and it's very likely that he'll stabilize if he's got a level or two in something with decent Fort. d20 Modern nonlethal damage DOES allow for someone to be knocked out with one shot, regardless of hit points -- which means that a stealthy ninja-type could drop down from the rafters and knock out a 10th level goon with one hit.
In short, in d20 Modern, going into the negatives from lethal damage is easier, due to the Con damage threshold, but less dangerous, because of the Fort save to stabilize (although still a hassle, given the lack of healing). For someone who wants to knock people out without special feats, the damage you do in normal melee can take care of this. Knockouts should only be taken by someone who wants to specialize in knocking people out without doing them any long-term hurt -- and if you purchase your feats properly (you know, the ones you get at every even basic class level, every third character level, and so on?), you'll be in fine shape to knock out mooks whenever you want.
I don't think that nonlethal damage is BETTER than subdual damage, mind you. It's just that they work well for different games. I wouldn't want to play a D&D game where my characters could be one-punched out by an ogre even if they were 9th level, and I wouldn't want to play a d20 Modern game where a 6th level guard with 40 hit points couldn't be knocked out, but had to be battered down over the course of several rounds, blowing his alarm whistle the whole time.
Does this help? It's really just a different mechanic -- making people fragile and using fewer numbers...