Re: The Joker's background?
Joker's one of the few villains I can think of that truly benefits from having an anti-backstory. He's such an over-the-top, theatrical character that it's always a bit hard to swallow that he was ever a relatively average mob enforcer/crook/comedian/whatever. When you remove all of that (or at least make it very vague) it liberates him in a sense, to be less of an identifiable man and more of an anarchic counterbalance. Plus it plants for firmly in opposition to Batman, who of course has a very detailed and rich origin.
O Captain! My Captain! Rise up and hear the bells.
Rise up, for you the flag is flung, for you the bugle trills.
For you bouquets and ribbon'd wreaths, for you the shores a-crowding.
For you they call, the swaying mass, their eager faces turning.