FINALLY saw this.
Yes, Heath is unbelievable. He completely melts into his character. You see the Joker, not Heath. He's a blank, a cipher, you have no idea who he is or where he is from or what his motivations are. Frighteningly, it is suggested that he has none. He's an ID personified and he is portrayed as a terrorist, wanting nothing more than to spread terror.
I also agree the rating was too low for the movie. The movie was dark, the violence extremely disturbing.
I'm not sure why they insisted on keeping the Dent character plotline. They killed him at the end but left the Joker alive, yet Aaron Eckhart is still with us and Heath is not.
The explosive danger was fairly unbelievable. It's supposed to invoke 9-11 I guess, but if you think about the reality - how many drums of oil were needed? And NO ONE saw them loading up every single floor of a hospital? NO ONE saw them loading them onto a couple of ferries? Ludicrous - as are people falling 30 or so stories and living. And both Dent and Batman fall 3 stories, but Dent is dead and Batman lives? Why is that? His suit can protect him from bullets and such, believeably, but not such severe carwrecks and incredible falls. Yes, the wall of monitors sparking and crackling when Freeman's character signed off was lame. I was expecting a countdown of some sort and the screens displaying the destruction of the information, not literal destruction. Batman shuts down his entire operation, destroys all his records, he's going to turn himself in. Yet he can get it all back up in minutes. The fact that a firetruck can be on fire in the middle of the city and blocking a road, and an entire police station is blown up and neither is reported, nor draws a crowd is also way out there.
Plus, how did Batman know where Dent and Gordon were at the end?
Yeah, some parts are very comic-booky, but if you ignore those, it's very enjoyable.