For me Gran Turino was this - Clint Eastwood was, well, Clint Eastwood - ******* awesome. The rest of the characters (the Mung family for example) were very poor actors. They wanted an authentic Mung family and traded that in for acting ability.
Dark Knight - I agree, Ledger's performance was off the charts. But take him away and what do you have? Now, I understand, it's not fair to judge a movie based on the best character being removed. I just found myself thinking after the movie, "damn I wish there was more Ledger in it."
But then, I had the same thought after Brokeback Mountain, what do I know?