Like many of the actors in Nolan's Batman movies Ledger's previous recognition re: awards and nominations had more to do with whether or not he was right for the role. I personally felt that his dull and lethargic take on the character completely missed the mark and I truly believe that there would have been a "Phantom Menace"-style divisive backlash if he'd lived to see the movie.
The "I made my mind up 6 months ago!" camp would have shown blind love for the performance but I think there would have been a significant chunk of the audience that would have seen it as a terrible miscast.
Ultimately I think Ledger was a talented actor who had a career of wildly-varying quality, and for everything like "Brokeback Mountain" that showed actual talent and skill and real promise in regards to eventual greatness there was something like "A Knight's Tale" to show that he just wasn't the kind of presence that could redeem a crappy movie.
Like most actors he was fallible and at the mercy of the project itself and I don't think his phoned-in performance is even close to redeeming "The Dark Knight" nor do I think "The Dark Knight" is good enough to soldier on with casting choices that were ultimately poor as far as I'm concerned.
I know what kind of responses I'm gonna get from this post. You can't criticize this movie or his performance online without drawing out the crazies, I just don't care. I'm entitled to my opinions.
Also, I take objection to people putting their own words and thoughts in the mouths and minds of the deceased so I'm not going to sit here and tell you what I think he would think of all this unwarranted TDK hype but I will say that there are quotes from Gary Oldman and Ledger's best friend/agent that suggest that Ledger didn't take the role too seriously or give it much weight. Per his agent he was hoping to get fired because he'd get to pocket his paycheck and have several months off (seriously, you can find the quotes in a Vanity Fair article) and I think that shows in the movie.
But hey, what do I know? The internet myths that Ledger lost himself in the role were going around before his body was even cold and that kind of hype has more to do with this particular performance's status than the performance itself does.
Flame away, crazies.