One commenter mentions the scene where Don's daughter shouts at her parents because her grandfather is dead, and they are not reacting emotionally. They sit at the kitchen table like hollow people. She goes into the living room and the telly is on. On the screen a Buddhist monk is burning alive.
Perhaps the most famous scene is Don pitching the 'Carousel' idea to Kodak. What is good about it is the acting, style and script, which are marvellous. But the commenter correctly identifies what is great about it: throughout we do not know whether Don has achieved a kind of creative enlightenment, where through his cynical advertising pitch he has located truth and goodness, or whether conversely he is devouring and using his own life and his own values to win an advertising account.
I think this ambiguity is what I want most from art, because it is a true feature of life. There is no final revelation or settlement which shows what things 'really' are, or whether things are good or bad. I think the best stories are ones where the key question is sustained and open throughout. Like: in the old Scooby-Doo the ghost was not real, and in the new Scooby-Doo the ghost is real. But in some uber-Doo in my head the kindly old janitor never takes his mask off - or I suppose, he does strip it off, but we still don't find out what is underneath.