Originally Posted by BatLobsterRises
Then you're getting more into the plot mechanics in a discussion about characterization.
I can't really think of a good reason to justify how and when The Joker wired an entire hospital to explode, but ultimately I don't care because seeing The Joker walking out of a hospital in a nurse uniform as it blows up behind him is one of the most Jokery-things imaginable.
It is part of characterisation.. as the film is possibly asking me to see Joker as some mastermind planner down to the last detail. He can even time when to drive out a school bus at the exact second when a line of school buses are passing (and they somehow don't notice that one bus is driving out of a big hole at the front of the bank). And he times everything perfectly so Batman can arrive in the nick of time to save only Harvey Dent.
I still think Heath Ledger did have a lot of good moments though.