8. He Never Looks At His Victims
Some things about the Joker have an impact on you without you even realising what exactly it is thats so disturbing. Sure hes crazy and the calm moments are accentuated by those brief explosions, but what really nags is just how casual he is.
Look at his kills throughout the film; he hardly ever looks at anyone as he takes their last breath (and even when its not directly at his hand, like the exploding phone, he stills looks away from the carnage). This isn't some trait simply imagined up for the film and neither is it a key proponent from the comics; the whole notion is grown out of a throwaway reference in the source. From the comic run The Man Who Laughs (named after the film that directly inspired the original version of the character), the Joker is described as being so ruthless he doesn't care to look at people at he kills them. Such a key part of the villain from such a hidden source.
Im not too sure whos idea it was to include this, but the execution - incredibly non showy - is pure Ledger.