Right at the start, I expect better of the Joker than to be mugged by kids, that should be an alley full of dead punks. Joachim Phoenix looks like crap, he's 45 here, at least two decades too old for this origin story, isn't really fit for the job. Maybe he'll toughen up by the end of this, but physically? Nope.
I definitely prefer the Jack Nicholson take: Hardened criminal who THEN goes crazier. I don't mind The Killing Joke story where a comedian becomes Red Hood, then goes crazy/gets dunked to become Joker, but his skills there are implausible. And none of this Jared Leto shit.
The plot's generally more pathetic downward spiral than an arc up to criminal mastermind, even if he has winning days (the well-memed stairs scene and on), he's a joker, not The Joker, absolutely not Harley's "Mistah J". I can't see this guy learning to make bombs, recruit hoodlum henchmen, rob banks, fight Batman, run the Legion of Doom with Lex Luthor (who's the kind of rich bastard this version would hate).
But the crimes and riots here are a pretty good start. Phoenix's Joker has a kind of pathetic charisma, good for attracting broken losers (which is all of us, except the pretty rich people, really). The cult of The Joker could be born here, even if the actual figure's not that good.
For once the Hollywood inability to leave movies alone without a fake color grade works, every scene is recolored something crazy, matching the Joker's moods. It goes from a not-terrible imitation of '70s film stock, very very obviously copying Taxi Driver, to Suspiria levels.
The supporting actors are meh, and they're a lot of the problem.
Robert De Niro as fake Johnny Carson is ancient and decrepit, he just looks bad, and of course De Niro is the least funny person in the world, it obviously hurts him to even smile, so faking Johnny's impossible. They should've got an actor who isn't mummified to play the role. Johnny retired at 66, De Niro is shockingly only 76 here, but he looks 96, barely held together by a suit.
Joker's mom (played like the Cryptkeeper reanimated Mother Bates) thinks Thomas Wayne (played incompetently by TV extra nobody Brett Cullen) is going to help her for personal reasons. In the movie, he's just indifferent, an out-of-touch Trump-like asshole, but this is a point I always come back to with the whole Batman mythos:
The Waynes made Gotham, and Bruce perfected it. It isn't that way—garbage piled up, grafitti on top of grafitti, poverty everywhere, corrupt and useless police, protests, steam pipes and vulture-like gargoyles looming over all—because they don't care, it's that way because the Waynes want a city full of despair and freaks, so that the older generation of Waynes can pretend to be philanthropists (much like Bill Gates does to the tech industry), so that crazy little Bruce can swoop down on villains in his multi-million-dollar battle armor and practice his rich-white-boy ninjitsu. So if terrible shit is happening to Joker's mom, it's because Thomas Wayne wants it that way.
I'm particularly unimpressed with Alfred (Douglas Hodge) and Little Bruce; Alfred's not supposed to be a wuss, he doesn't hiss and whine about cops to strange men at the gates, he's (depending on iteration) former British Army, and while he's not The Batman himself he's more than capable of defending the manor. The version in the Gotham TV series (played by Sean Pertwee, son of my favorite Doctor Who) is so much better. Little Bruce is a puppet even by the standard of child actors.
Slight amusement: The movie theatre at the end, where Batman is made… is playing Zorro the Gay Blade.
★★★½☆ — Kudos for trying something different, if not always successfully.