but here is my question: why does the Joker have to be scary? it's not a horror movie, the 89 Batman was a comic book movie that was mainly targeted towards kids. he didn't have to be scary, just ruthless (well, comic book ruthless anyway).
The Joker is demented, he's sadistic, cunning and ruthless, he'll never attempt to kill Batman, because Batman is his equal, and as you saw. ALL of that was portrayed brilliantly in TDK.
The Joker is so crazy he's scarry, get put in a room with a crazy person who is unpredicatable and you tell me you wouldn't be frightened?
All of that isn't "comic bookie" because in the comics he murders for almost no reason, he'll cause harm and chaos for the same. He's not some over the top character as portrayed by Nicholson, but I also give blame to the writer as well.
If the Joker is not scarry and ruthless than that's not the character and you missed the point.