Heath Ledger and the Joker is what makes Dark Night a better movie, in my opinion anyways (man). It's not simply who the actor was and who the character is, but what they all brought to the movie. With Begins, you see Bruce/Batman fighting against corruption, a nice variant of good vs. evil.
With Dark Knight, we're introduced to an agent of chaos. Someone unlike common thieves, murders, and even the big shot crime lords. Someone who wants to see the world burn, even if it means they might die in the process. They don't care, and hell if they truly care about anything (pride, money, turf, etc). What tops it off is that Joker has a lesson he wants to teach all of Gotham. Like in the comic "The Killing Joke," he wants to show that anyone can become as insane/mad as him and that all it takes is a little push. This could be having cops kill hostages, rigging ferries to explode unless one ferry blows up the other, and of course turning the white knight of Gotham into a deranged murderer.
In the end, no one truly won or was correct. While none of the ferries blew up, proving people are beyond selfish savages, Harvey still succumbed to hatred and revenge. To protect the image of hope and peace in Gotham, Batman must dawn the burden of being Harvey's murderer and be chased by not only the authorities, but the citizens of Gotham.