The only reason he’s scary any more is that he’s got the writers and the marketing people at Warner on his side.
Since he has the writers on his side, he can escape from any prison, no-one EVER thinks to just shoot the bastard, he’ll never be executed despite the tens of thousands of murders he’s committed, etc.
Realistically (inasmuch as anything in comics can be realistic) he’d have been extradited to say, Texas and executed, or some Gotham citizen would look out his window, see him pushing his henchman in front of a truck and just assassinate him.
And given his death-count and the given fact that no cell can hold him for longer than he wants to be held, Batman should have just snapped his neck a long time ago.
There was a story where Bats had the ability to let the Joker die as a result of a failed scheme of his–not kill him, just not actively save him (I don’t remember the details–perform CPR, put a tourniquet on him, administer an antidote…something like that but more complicated and time consuming, IIRC) and Gordon is saying “Don’t–just use the time you’re saving him to stop a robbery somewhere else–stop a murder. Break up a drug deal.” and instead, Batman wastes his time saving the Joker. That’s when I stopped being interested in the Joker.
Another one had the second Robin (who was beaten to death by the Joker–but got better (don’t ask)) about to shoot the Joker. The second Robin had Bats tied up with one arm free, gave Batman a gun and said “You have three choices: do nothing and I’ll shoot the Joker, shoot the Joker yourself or shoot me.” and Bats chose to shoot (ok, throw a razor sharp batarang into) Robin rather than let the Joker die. This is just bad writing.
HG Wells said “If anything is possible, nothing is interesting”. Writers have been going to such idiotic lengths to make the Joker immune to everything that he’s not scary any more. There’ve been stories about the Joker vs Superman or Wonder Woman or Green Lantern or The Spectre–and the Joker always gets a stalemate at best when he should end up as a pasty white smear on the pavement. Used right–a very hard to capture, very elusive Batman villain who randomly kills (ala Hannibal Lector) and who strikes from the shadows, he’d be scary. For him to take on the entire Justice League, to hold parades in public, to taking over the UN building, to getting in fights with The Spectre? Meh.
A Joker who’s smart enough to NOT take on the Green Lantern Corps or Superman would be scary. A Joker so stupidly overpowered that he CAN take them on doesn’t make him scary, it makes him boring.
The only thing that makes him dangerous any more: he’s popular and marketable and the writers will warp the plot and make all the characters idiots to make sure that he stays marketable.