6. Pain
In case you missed it, the new Joker's scars are just as important as Ledger's or Nicholson's: they just happen to not be on his face this time. Joker's body is a map of his achievements and his pain; it's important to remember that everything about his appearance (particularly that iconic face) is a mark of the agony that's turned him from a presumably "normal" guy into the kind of person who beams joyfully at the prospect of making someone else hurt "really, really bad". Leto has spoken quite openly after filming his scenes that playing the Joker was a painful experience, and it is increasingly appealing to think of this new version of the character not as a agent of chaos, or one of vengeance, but as an electrified raw nerve. If he has the same sort of agenda as in The Killing Joke, that would be the perfect analogy. It would certainly be a more comfortable analogy than Leto's statement that playing the character was like "giving birth out of my prick hole".