Perhaps I just don’t remember the original Joker well enough: I remember Joker doing the playful bits, like the televeision commercial for the Joker vecom (Smiley?), the hand-buzzer frying of the guy in the board room, or the ‘slashin’ to the oldies’ in the art museum’ bit. The sort of high, manic Joker that’s probably best suited for younger audiences. This seemed much more like the sort of joker who would kidnap Jim Gordan just to show him pictures of a naked, shot-up Babs Gordon.
Perhaps it’s that he’s manic in his sociopathic killings. The guy really does seem to enjoy them, in a way that other killers very rarely are featured in doing so. Or perhaps it’s because in the Joker’s own strange way, he’s very sane. Zsasz kills as part of a compulsion to fill his body up with tally marks. He has some sort of motivating factor which drives him to murder. The Joker doesn’t hear voices in his head telling him to kill people, nor does he fail to realize what he is doing to people (like, say Grundy). If the Joker has any sort of motivation for his killings, it’s to piss off Batman. I don’t know… what aspects of the Joker do you think are key to his character?