I don't know why the critics are against the Joker when his first act of murder was shooting those three Wall Street guys. Did they not see them harassing the woman on the subway? Also the imagery from the protesters was right up their alley: "Resist" signs, "Fascists" signs, and even an effigy of Thomas Wayne that looked more like Trump.
While it is confirmed Joker is not Wayne's son, revealing that he was adopted and was abused by his mother's boyfriend adds to the nature vs. nurture debate. Prior to that pivotal scene you may lean towards the nature aspect of the Joker's mental state because his mom has issues. But maybe this could be a mix of the two. It also makes the part where he smothers her that more depressing since you've seen the sweet moments between the two but also find out she did nothing to stop the abuse. The mood definitely changes when he calls her by her name and string instrumentals start up. I agree with others who say this score really adds to the movie.
Personally I don't think going back to the previous scenes with Arthur and his neighbor were needed to reveal they were not dating. Since it was pretty obvious when he broke into her apartment, there never was a relationship given how she reacted (plus when you go back to the diner scene, the way the cook stares at them indicates that Arthur is sitting alone but acting like he is with someone). Still not everyone can put things together and they'll need to be told what is going on.
As
@Alto stated, this movie can work as a standalone. Sure you had the Bruce cameo and the murder of his parents but maybe Warner Bros. should think twice if they want to start another DC cinematic universe in a different direction (I don't know if the upcoming Batman movie that is suppose to be more of a detective movie will have tie-ins to
Joker).
On a final note, I like how the final scene did not show how Joker killed the doctor at the asylum. All you saw was the bloody shoeprints and him being chased but it gave you implication that he could easily escape if and whenever he wants to.