good grief !!
yes, he was brilliant as the Joker, yes he brought the more evil and psychotic side of the Joker to life
but before him, i'd have said that Jack Nicholson's version was perfect.
i don't see why the character should be made to suffer just because the actor kicked the bucket
if you're going to retire the Joker from the screen, then retire Batman too - the whole point is that they need each other to define themselves, it's the way the characters evolved in the comics. as the previous movie franchise proved, if you kill the Joker off, the series is going to do downhill.
IF there is a 3rd and maybe 4th Nolan film, then yes, they shouldn't use the Joker as the main villian, but maybe a cameo at the start perhaps.
and then, in the 5th film, do the Joker/Scarecrow team-up from Knightfall, by which time all the fanatical fans will be fixated on some other poor actor and have forgotten all about Heath Ledger