I think I am best qualified to answer this question.
Paul Bettany would make a perfect Joker for many different reasons.
1. The guy can do sadistic SOB. Unless you are British or simply a fan of oft overlooked British gangster films, then chances are you never saw Gangster No. 1. Its a shame - fantastically brutal and interesting movie staring - you guessed it, Paul Bettany.
Paul Bettany played the main character who became increasingly evil as the film goes along. The intensity of hatred Bettany is able to portray with his eyes, on top of the pure energy he brought to many of the torture scenes simply screams Joker. In fact if the character had charisma and green hair, he would be a perfect Joker doppleganger.
2. The guy oozes charisma. Take away his sadistic 'Young Gangster' portrayal and you have a laundry list of roles putting Mr. Bettany playing a charismatic man. A Knights Tale showed his ability to become a show man in character, Firewall showed how he could be a charming con man, interviews on late night talk shows showed his wit and humor. Combine this with number one and you have Joker in the flesh.
3. He respected the character and Nolan's previous work. He went on the record several times about how much he wanted the role, how much he liked the character, and how much he would love the chance to work with a director the caliber of Nolan.
4. He looks like the Joker.
5. I said so.