It's pretty unfair to Ledger's performance to say the reason it gets such acclaim is because he died. People do gush about his performance, and the is a kneejerk reaction against the Ledger fanboys...but independent of that, it's like he can't be fairly judged for his performance.
Maybe if he didn't die, there might not be so much gushing over his performance, but for what it's worth, I knew I was in for something special when I heard "you're just a freak...like me" in the December '07 trailer, before all of the added interest since his death.
It was also nice to see him get the oscar. I loved to see "comic book" movie win for something besides the typical production design, special effects, or other techinical award. Some people will say it's just because he died and there were more deserving actors that year, but how is that different from the debate that occurs when anyone wins?
I have strong feelings on the subject too.