I love the ferry scene and how it played out. Everyone on the citizens' ferry was saying they should push the button. But who had the guts to push it? None of them. There wasn't just criminals on that other boat. There was Cops, prison officers, and the ferry crew, too. Nobody wants to live with that kind of slaughter on their conscience. As for the convicts, one noble convict took the decision out of their hands.
My favorite part was how Batman was the only one who had any faith in the people to not kill each other. Joker was certain they'd do it. Gordon thought they would. It was Nolan's version of Joker's sick experiment on Jim Gordon in The Killing Joke, where he tried to push him over the edge to prove anyone was as bad as he was when push came to shove.
Batman essentially says the same thing to Joker when the ferries don't blow up; "What were you trying to prove, that deep down everyone's as ugly as you? You're alone".
Loved it. It made Batman's sacrifice for them at the end of TDK more potent, because he knew Gotham's people were still ready to believe in good. He didn't want them to lose that by losing faith in Harvey Dent and all the good work he'd done to help their city.