After reading reviews for this film, I found it difficult to distinguish reviewers' annoyance with the film itself from its promotion on social media, referred to, cautiously, as "discussion" and "conversation." Having seen the film, though, I understand their displeasure. Joker isn't very good.
Joker is comparable to Batman Begins, the first film in Christopher Nolan's Dark Knight series, wanting to be a kind of psychological investigation, an answer to the question of how this villain comes to be who he is. It's an interesting "take" on the superhero film -- where the superhero protagonist is presented as a real person, so to speak.
The Dark Knight series questions the idea of the hero. Are our heroes wholly virtuous? The answer, of course, is no. Batman, or Bruce Wayne, is the dark night, not the white night. He makes, arguably, villainous decisions. So if the hero makes villainous decisions in the Dark Knight series, we might expect the villain in Joker to make heroic decisions. Unfortunately that's not the case here.
Arthur Fleck is mugged and jumped. He is the victim of child abuse. He is the butt of the joke on a late night talk show. Such scenes, it would seem, are intended to drum up sympathy for our villain. And perhaps they do -- which I believe is what the topic of social media discussion -- but it only works to a certain point. The emotional work is much more than putting a bandage on a puppy: we might feel bad for the puppy but the bandage doesn't make the puppy any more compelling. It's weak, incomplete characterization.
Why incomplete? Arthur Fleck lacks what workshop and craft discussions call "redeeming qualities." He lacks the intelligence and appetite for mayhem of the Joker that we've seen elsewhere.
One of the better scenes in the film, I thought, was when detectives pursue Fleck on the subway. Fleck hits someone in the back and runs to the next train car. When the person turns around, he sees the detective and assumes the detective hit him. A brawl erupts on the train. This, if it weren't unintentional, would be the kind of trickery we expect from the joker.