
Let me start off by saying that Heath Ledger is guaren-dam-teed to win the Best Supporting Oscar for this, postumously of course.
Ledger in the role of The Joker was, in one word, brilliant. He deserves all the awards and adulation that he recieves for this. Even for ones he doesn't. His role as the Batman's arch-nemesis was the best EVER! With all due respect to Jack Nicholson's performance in 1989 (which was a different role, different movie, different time) no one can ever compare to the Ledger performance.
From the make-up (awesome) the voice, the laugh and everything in between, I can never read another Batman comic and not think of Ledger ever again.
If Ledger had not passed on, I could have gladly and easily watched a Christian Bale/Heath Ledger marathon of follow-up movies. Just those two going at it and I'd be more than satisfied. This movie was the most true to any comic-based characters ever and for evermore. That includes Spiderman, Superman, and the X-Men.

Maggie Gyllenhaal was a throw away. The producers should have tried harder to sign Katie Holmes for a reprisal of Rachel Dawes. Apparently she wasn't interested this time around. I'll bet she's kicking herself now, seeing that the world-wide gross for The Dark Knight is currently at $991,822,235 and counting, and sits and #4 all-time, according to the Internet Movie Database.
The only real downfall of this film was the voice of Christian Bale's Batman. It was disappointing to say the least, if not down right goofy.

An instant classic. It deserved and lived up to every bit of anticipation and hype it created, and then some.
By the way,
"Why so serious?"