Chaos is his motivation. The Dark Knight posed the question of how to fight terrorism when the world has nothing the terrorist wants except for fear. I get what you're saying, but I think that's far more interesting than just another cheesed-off gangster stereotype. Comic books have always tried to mirror the times we live in and Nolan really succeeds in TDK. Is it overrated? Well, sure. But I also think it was one of the best films of 2008.
I never liked the assumption that when you finally get a worthy reboot of a franchise all it does is turn off "purists" who can't see the difference between a worth while project with a lot of thought and care behind it and something to the extent of New George of the Jungle or something.
That said, sure I get the "overrated" bit, but remember WB has been struggling with Batman's franchise for YEARS after Schumaker screwed up his films. I was hearing rumors in the late 90's about a 5th Batman movie with Marylin Manson as Scarecrow. Thank Frog that never happened.
Did Ledger's death help Dark Knight out? I dunno, but the first Batman Begins film didn't do all that well either. And I've said it a hundred times, DUDE... they should have taken the same chance with TDK for Superman, which died after one movie.
As for the terrorist style Joker, I think it was a pretty brilliant version of the character, as was said, not making him a cheap gangster. The mystery behind why he's doing this simply revealed as he's doing it because he's crazy actually has some levels to it, if you think about it. That said, who can resist Mark Hamil's Joker from Batman TAS? The perfect balance of looney and crook without being too cartoonish or too dark. Of course, I didn't really care much for the similar style of the Joker on The Batman, fueled by just being crazy and having no real motivation either. At least at first. Though I gotta say, The Batman's Riddler was a bit better than the TAS version.
Of course, let's not forget TDK had Twoface in it (though not mentioned by name) as well. And again, watch Batman Forever. I HATED how they turned the dark, nasty character with a ruined life into essentially a surrogate Joker in that one. Jim Carrey's Riddler I can sort of understand (bad impersonation of the 60's version and all), but Daffy Duck Twoface?