Does it matter whether you use lethal or non-lethal tactics in Deus Ex: Human Revolution?
According to the stealth tutorial video, a lethal takedown makes more noise than a non-lethal one.
Neutralizing an enemy without killing will award you a +20 XP bonus ("Merciful Soul").
Knocked-out hostiles can be woken by other people who happen to find them. Obviously, this can't happen if they're dead.
I'm not aware of any impact on the story, though there is a Steam achievement (Pacifist) for not killing anyone except bosses.