The Anti-Climax of the Low Chaos Final Boss. You've infiltrated the most heavily guarded areas to assassinate the richest, strongest people in all of Dunwall. He knows what you're capable of and there is absolutely 0 possibility of making it in a fight with you. Confront him directly, and he won't even resist, because he knows he doesn't stand a chance.
Pulling off a non-fatal 'neutralization' is just as satisfying, if not more so, than slaughtering your targets. It's one thing to kill a person, it's another thing to murder them gruesomely, and it's still not as good as making their lives a living hell for the rest of their miserable existence.
You can take the High Overseer and brand him with the mark of a heretic, ensuring that he will wander the city forever, disgraced and shunned, to the point where it's against the law to help him at all. Plus, when you're in the Drowned District, you'll find him as a weeper, with a note cursing Corvo with his last words.
The Pendelton brothers can be kidnapped, shaved bald, have their tongues cut out, and be forced to work forever in their own mines as slaves.
Lady Boyle can be sent away with another Lord who truly loves her and insists that she'll love him too...eventually. No matter what. Granted, it's not as harsh as the others, but much more creepy.
Actually you can get a far more detailed account of her fate, if you meet the Outsider during the same level, he says he sees both of Lady Boyle's fates: In one, she dies, in the other, she's forever locked away, miserable, as her dresses turn to rag and her hair grey. "Either way, this is Lady Boyle's last party."
The Lord Regent Burrows has his plan to spread (another) plague across the city broadcasted far and wide, sending him to the prison for summary execution.
If you've done all of the above, or just kept the chaos down, at the end. Havelock has a Villainous Breakdown, kills the other two Loyalists, and pretty much just waits for Corvo to come finish him off, pathetically babbling to himself. Just the threat of you is enough to break the spirit of any man who has done immoral things.
It gets to the point where, by playing a no-kill run, even The Outsider is impressed with what you've done, calling you 'fascinating.'
During the mission to take out Lady Boyle, you can come across a guest book, with the option to sign it. Doing so and then reading the book reveals that Corvo signed his own, real name. The amount of steel in Corvo's balls is out of this world.
Not to mention the fact that he goes to that masked ball 'as' a wanted man who is systematically destroying the government.
Not only that, but if the remaining sisters are not killed they will actually thank Corvo for sparing their lives and will give him a gift after reading the guestlist.
If you do that, when entering The Lord Regent's chamber, you can find a letter about Lady Boyle's death/disappearance. The Watch noticed the guest book signature, but are completely at a loss as to how it got there, assuming it's a fake from a noble with a bad sense of humor.
It's possible to take out a master assassin and an entire building of his Elite Mooks without being seen, using only a knife and a couple of your low-level powers.
Even better, you can sneak past them all, without being seen or drawing a drop of blood. Daud will even praise you for it. The nonfatal method of neutralizing Daud consists of merely stealing the few coins he has in his pockets, with an implied Villainous Breakdown when he discovers how easily Corvo could have killed him.
Piero and Solokov get one when their combined research defeats an entire army outside the Hound's Pit pub.
Doubly so when it cures the magical plague afflicting the city. Even the Outsider is impressed by that one.
Even better, you can just kill and/or render unconscious all the guards on your own. Piero will compliment you on this, remarking that he had spent weeks working on something that you were able to handle on your own in minutes. Though he still sounds a bit ticked off that you were able to.
Samuel (of all people) gets one in the High Chaos version of the final level. He calls you out on how much you've embraced your role as a sociopathic, bloodthirsty murderer. He tells you you disgust him and demands that you get off his boat. And then he shoots off a flare gun to alert the guards - and even tells you he's doing so beforehand. He basically "betrays" you to your face, showing that he has the courage and integrity that the other Loyalists lacked. And, again, he's saying and doing these things to the man who just single-handedly murdered pretty much everyone he saw. Samuel may not fight, but the man has got guts.
While assassinating Cambell, your options to neutralize him are large. Poison him by switching glasses, marking his face with an iron prod so he'll be exiled to plague filled streets, or simply throwing a grenade at him in his own secret claustrophobic room.
Lady Boyle has just as many.
Daud going One-Man Army against the overseers in the Final level of the Knife of Dunwall.
In the flooded district, at one point you can come across a couple of assassins. One is instructing the other. He tells him to cross from one side of the room to the other without being seen. You can, quite easily, follow the assassin-in-training across the room, without being seen by either of them, and leave without them being any the wiser.
Sabotaging Delilah's ritual, instead of performing a Grand Theft Me on Emily, she gets sucked into her own painting. A Void within the Void. She begs for the Outsider's help as she's being sucked in, and curses Daud's name as she loses her grip. You get the feeling that Daud, the player, and the Outsider are all laughing at her as this happens.
Even better, if one thinks of Emily as the Empress, this means that Daud is a better Royal Protector than Corvo.