Expertly crafted, but needed more action
Even taking into account that Todd Phillips and Joaquin Phoenix were going for more of a Taxi Driver-esque character piece, there was still too little time spent with JP in Joker mode.
I kept waiting to get to the part of the film (the entire second half, I assumed going into it) when we'd get some full-on Joker action where he terrorizes the city with his minions, and that just doesn't happen. We never get there. It's called "Joker" but it felt as it this entire film was ultimately a big tease for a sequel where we actually get to see a story focused on the fully-developed version of that character.
It's rare that I think movies need to be LONGER than they are. Usually I am the one saying that a film could've been trimmed down. But in this case, I thought there could've been another hour and the film would've been much stronger for it.
At the moment when the Joker is standing on the car, in the middle of the clown mob, it felt to me that that was the beginning of a new act of the story, where we finally get to see the Joker break bad on the world . . . but then it was over.