others did it better

Rating: 7/10 (10 October 2019)

That's the TL:DR: others simply did it better. Both the "man reaches breaking point and becomes violent" / "slow descent into madness" and the Joker himself.
The long of it is: this is by no means a bad movie. Neither is it mind-blowing. Given it had the best ingredients in the world to work with, aka Joaquin Phoenix and a nice cast all around and obviously wonderful art direction, costuming etc, it was a serious let-down. Especially since it so easily could have been great. So yes, I rate my expectations.
I also rate my emotions and cringing in shame on behalf of a person for 2 hours is precisely not how I want a joker movie to make me feel. This is the sad sack loser joker nobody asked for (before and after the supposed transformation). There is nothing cathartic about seeing him beat random people up, you just cringe even more at his delusions that everybody is personally out to get him.
To be more specific: the characters arc did not fit with the narrative they were going for in my opinion and the portrayal did not ring true to the "Joker" actions for me. Too much sadness, too little repressed rage. It didn't help at all that we got overly dramatic music to underscore every moment of defeat instead of letting us take things in quietly. The music killed most of the important moments for me. I also consider it a sign of the movie needing to cue in its audiences to what they're supposed to feel, like the flashback reiterating the big reveal that was already made clear in the dialogue between the two people.
There was also, as others have pointed out, nothing funny about the movie. While humor might be subjective, I've always been a fan of gallows humor and as somebody who was laughing and crying at myself simultaneously because my pitiful situation was just so ridiculous, I thought I would be able to sympathize with Arthur when he realizes that his life is one big joke. Nope, not a chance. See, Arthur is not the (awesome!) Comedian. Jeffrey Dean Morgan perfectly embodies the sentiment they were going for in 30 seconds where this whole movie can't manage it. Arthur takes himself very very seriously and he can't stand people laughing about him, much less is he able to do it himself. So how does that fit in with everything being a joke to him? Not at all, obviously! Because we can't have an accurate portrayal of this one person's mental health, we need to shoehorn in some "typical" Joker actions and statements that are at odds with the character. In ascribing them to this particular character, they are also robbed of the exact flamboyance and joy they should normally transport. Yes, joy, or at least satisfaction. The joker in my favorite iterations (Ledger and Hamill, though the new game voice is in line with the sentiment) clearly enjoys what he is doing. No joy to be had in this movie though.
The only legitimately good moment was when the whole audience in the cinema started laughing when Arthur killed Randall. Problem is, the funny thing was the pointlessness and the absurdity of it all in context, so it was a laugh at the expense of the movie and not one of approval.
My personal view: save yourself the ticket money and 2,5 hours of a torturous journey that goes nowhere, both personally and narratively and re-watch Ledger or your favorite animated episodes.