The movie title should have referred to the Joker because that's what it's about.
To better understand what I'm about to say you should know that I'm a Batman fan. And I'm most likely a bigger Batman fan than you are. (If your first response to that is "No you're not. I am!" I'll concede to you. But then that would make you and I bigger Batman fans than most readers of this post. I won't bore you with my geeky credentials, just trust me.)
This is the Batman movie that mature (or should I say adult?) Batman fans have been waiting for. I thought it was "Batman Begins" which is awesome, but nope. The Dark Knight is it. "Finally", we grown-up Batman fans are thinking, "the rest of the world will understand why we like Batman so much."
This the Batman the world has been deprived of for so long by all the other attempts. Batman is not a superhero, so much as he's the anti-anti-hero. Batman, as we fanboys know him, is a dark, conflicted character that escorts us into a journey into human nature that is so scary that we're glad to have him as a guide, even though we know he'd never let any of us actually be friends with him.
I kept asking myself if I liked the movie throughout the movie. I realized when it was over that I should have been asking myself, "why so serious?", which is the promotional tagline of the film, and is spoken by The Joker. I was so serious because this is a serious film. It is not a summer super-hero popcorn flick. It explores the dark side of human nature in such a way that forced me to ask "where is the hope?" when I left. There is an answer found in the self-sacrifice of some of the characters of the film. But wow, what a journey.
This is the best-crafted comic book movie ever. Those of you who argue with me on that point will probably say that The Dark Knight is not a comic book movie, so "Spiderman 2" is safe on the throne. And you're probably right. Except that what most of you average moviegoers don't know is that this IS the comic-book Batman. This is the Batman in comics now. Dark, scary, mature. (Go read Batman: Knightfall, the novel by Denny O'Neil and see if I'm not right.) But this is definitely not a "comic book superhero" movie in the same way that Superman, Spiderman, Fantastic 4 or even X-Men are.
Ebert said:
Batman†isn’t a comic book anymore. Christopher Nolan’s “The Dark Knight†is a haunted film that leaps beyond its origins and becomes an engrossing tragedy. It creates characters we come to care about. That’s because of the performances, because of the direction, because of the writing, and because of the superlative technical quality of the entire production. This film, and to a lesser degree “Iron Man,†redefine the possibilities of the “comic-book movie.â€
Yes.
Are you ready for the review now? I don't want to give ANY plot points away. So let me just say that though I haven't seen the "Saw" movies, I imagine that the stuff the Joker comes up with here would be at home in them. The only difference is that you don't see the gore. The camera leaves the result to the imagination, but you KNOW. The dialogue spoken by the Joker may be scarier and more disturbing than anything you actually see. (And some of the stuff he does out of view of the camera is slasher-flick evil.)
And about the Joker: Yes, Ledger is unbelievably...believable. This is one of the greatest villians to ever (is there a word for the opposite of "grace"?) the screen. Though I think he may have borrowed the lick lipping thing from other screen villians, when Ledger did it I was reminded of a demonic creature slurping the last remaining blood of it's victim. He possesses this character.
This is not a comic book movie where the good guy fights the bad guy and has trouble along the way. (Like the Spiderman movies) This is a movie where the bad guy fights the good guy, and the good guy is forced to look at what's bad in the human soul, including his own.
This is a movie that grapples with original sin and the nature of evil. What makes us good? What makes us bad? The Joker, like Satan in the book of Job, says that good people are only good as long as the circumstances are right. But if you change their circumstances...they'll be evil like him. His motive for his badness (and there's A LOT of badness) seems to be to force humanity to look at itself in the mirror only to find the Joker's face (as a symbol of evil) staring back.
Let me close with some comments on the other characters.
Bale is awesome as Batman. He undisputably the best Batman ever in the best Batman movie ever made. His performance may be overshadowed by the Joker's but it's excellence shouldn't be overlooked or forgotten that if he wasn't so good the movie would have failed. This movie was a team effort.
Maggie Gyllenhall as Rachel. A good performance by an actress with more gravitas then Tom Cruise's chickadee. It was necessary for the film's story.
Caine and Freeman as Batman's mentors/helpers. Awesome. The big three protaganists (Bruce, Rachel and D.A. Harvey Dent) all have to change in some way. These guys show that you can be solid supporting anchors without being 2 dimensional. They don't "change" but they feel like developed characters.
The people in this movie don't feel like comic book characters. They feel like people in an impossible situation. And while watching this movie you feel like it's totally plausible, which may be why it's a movie I appreciated more than I enjoyed.
"Why So Serious?" Because that's what kind of movie this is.
Note to Parents: if you take a CHILD to this you're as cruel as the Joker. Please don't. This is a movie by grown-ups for grown-ups. Those of us who grew up with Batman selfishly kept him for ourselves. Kids are going to have to settle for the movies and TV shows of the 60's and 70's. I'm really glad they made this move for me, but for my kids I'm sad. This is a Batman they can never see while they are still in the innocence of childhood.
There's a lot to chew on, so as I think of stuff, I'll add to comments here.

Do you know what a "Macguffin" is? Ebert defines a "Macguffin" as an object in a movie that drives the plot. It's what everyone is after and it doesn't matter what it is, or even if you know what it is like in "Ronin" or "Mission Impossible III"
Other filmmakers and critics define it as any object that drives the plot. George Lucas says that in the original Star Wars R2-D2 is the "macguffin". So I figure the ring in "Lord of the Rings" is the Macguffin.
So what is it in "The Dark Knight". I think it's the Joker's Plan. After you see it, tell me if you agree.