So, I've just finished my first playthrough, and I must say well done.
This game has accomplished more than I really had imagined it would.
So much so I'm already making my escape from the prison as I type this, in my second playthrough.
My non-lethal playthrough.
And that's where the flaw lies.....

We need more ways to dispatch of enemies non-lethally.
How about a trudgeon or the like? Drop-down knockouts perhaps? Hell, even let me throw my fists at them. I'm sure anybody who's played non-lethally, or just played for that matter, has snuck up on a a guard and had them turn around half a second too soon. Please, give me a way to knock him on his ass without driving my sword through his eyesocket. Sleeping darts don't grow on trees ya'know.

Also, if anybody can answer me this, how many can I kill before I get low chaos?
I've heard 20% per mission, but how many is that on each mission? Any answers I'd welcome.

My two cents, like it or not.

Ya, im still on my first playthrough, im most killing everyone but I have knocked out quite a few people and it really doesnt have a whole lot of options. But it doesnt bother me a whole lot.

It seems to me that the slow knockout was a very deliberate choice, because it made nonlethal something you have to earn rather than something that's just as easy as killing.

Remember how in Human Revolution it almost never made sense to kill someone? I'd imagine they wanted to avoid that -- and I'm glad they did. The game has lots of non-essential challenges baked into it, and taunting the player to really try hard on non-lethal playthroughs is one of them.

Having just started the second mission from your allies, I realized unilateral options for non-lethal would go through the DE:HR effect like RC said. Drop-down still would have worked though since it's contextual like the back having to be turned. If we had fists, we could just Blink and smash since we're divorcing non-lethal from stealth. I still think there should be a way to do that...non-lethal action, but I don't know how without diluting it.