Before you go to watch Joker, be aware that you're not going to watch an Avengers movie. This work has nothing to do with conventional comic books movies.
Firstly, be prepared for dark tones and sharp violence. Secondly, be prepared for spoilers here, as this movie cannot be reviewed without mentioning the focal points of its plot.

Joker is a journey inside the twisted mind of a failed comedian who turns into the most infamous super-criminal, but with a question, that only Joaquin Phoenix could submit so majestically. The question this movie tries to answer is: how could someone turn into the Joker in a realistic world?

Parallels With Taxi Driver

An older movie partly answered this question in 1976: Taxi Driver. Yes, you have many references to Martin Scorsese's masterpiece, from Robert de Niro himself playing the TV host Murray, to smaller details like the gun raspberry Arthur blows in the lift when he meets his neighbour and her daughter.
The parallels between Taxi Driver and Joker go well beyond the references you can find in the latter. In both films, the protagonist is a lonely man living in a dirty and morally decadent metropolis, who has been traumatized by life (Vietnam War for De Niro and child abuses for Phoenix) and that becomes desperate to prove he is alive.
When you are unheard, unnoticed, unsupported, unloved and with no apparent talent, violence can be the easiest way to undo your misery and make the world notice your existence.
That's exactly how both Travis and Arthur find their glory, although Travis in Taxi Driver fails to hit his real target, pouring his rage into a pimp and becoming a hero for the so called “good people”.
On the contrary Arthur in Joker, successfully kills his target, getting his personal revenge against Murray and the whole elite society, becoming then the hero of the forgotten, often regarded as the “bad people”.

Working Class Hero

Todd Phillips built up this contrast between the elite society and the lower classes giving a new light to the figure of Thomas Wayne. Bruce's father is always depicted as a wealthy, but generous man. Here the Wayne family is no longer idolized as angelic people, but as hypocrite plutocrats, uneasy when they have to confront “ordinary” people.
To give you an example of how powerful Joker is to this regard, think that I always venerated Alfred Pennyworth in every Batman franchise, yet in this movie he looks such an ordinary rich family's butler that I thought during the scene he appears in: “Come on! Choke that arrogant jerk!”
Thomas Wayne in particular, plays the role of the “two-face” politician, to the extent that he is almost the villain of the movie, while the Joker is ultimately elevated as hero! This is the main twist that makes this film insanely awesome!
In this movie Joker is not the villain, not even an anti-hero, he is the true hero of those people forgotten by the elite!
When we are kids we worship Batman, but when we grow up and life draws scars on our souls, we become fascinated by the Joker.

The other day I saw an article rolling on my Facebook feed saying: “This movie tells us that we are more similar to the Joker than Batman”. How true this statement is! I had the proof during the very screening of Joker, when a violent scene occurred. Here Arthur for the first time commits an unnecessary, out of the blue murder against his former colleague, causing the horror in his other former colleague, who is a defenseless midget. This poor little man is terrified, locked in the apartment of a psychotic murderer who is sitting on the floor in front of him few feet away, with his white face splashed with his then-friend's fresh blood. Luckily for the midget, Arthur still has a human side and remembers that the midget is the only colleague who always treated him nice. For this reason, Arthur allows the midget to leave the apartment, but... this man is too short to reach the lock... I'm a very sympathetic person, especially toward weaker people, so I felt very tensed during those seconds, desperately hoping that the midget could open the door... Sadly, the rest of the theater started laughing in an explosion of sadism.
You may laugh at this too, telling me “come on dude, it's just a movie! It's all fake!”. Of course it's all fake, but the reaction is real... And I believe as I can see it everywhere in the daily life, that there is still lack of empathy in our society. People don't care much about others' suffering or they are even amused by it.

Attention to details

As I mentioned before, Joker is very detailed when including even the most subtle references to Taxi Driver, but Todd Phillip's impeccable directing doesn't end here. What I enjoyed most is the director's smart use of lights. Photography means “writing with the light” and the team behind Joker took this definition to the letter. To give you a couple of examples I can recall when Arthur goes to a club for his first live performance as comedian. We can see only Arthur in the venue as the lights are pointing directly at the camera, so at us, leaving a dizzy darkness in the background. When Arthur starts his speech, drowned in his uncontrollable eerie laugh, we have the feeling that the club is empty. Again, the main source of light keeps us from seeing who's there. We can see the tables though and the red lights placed on them. There must be someone and the light is writing that the club's people don't enjoy Arthur, reason for which the light keeps those folks hidden from us. Then magically, as Arthur kicks off, the audience begins to enjoy and so the light decides to reveal those people to us... How to create suspense with a mere game of lights, without CGI involved!

Another masterclass use of light is directly linked to the plot as a whole. We see Gotham during the day-time as much as we've never seen it before and in some scenes we even see a bright sunlight! This strong sunlight is used when things turn better for Arthur, at least better from his point of view, like when he kills his mother or when he escapes from an Arkham room at the end. Light is there to depict the feelings of liberation and relief felt by the protagonist. We are immersed in this almost heavenly light that tells us Arthur's life is not a tragedy, but it's a comedy!

Music

There is no great film without a great soundtrack and Joker is no exception!
I didn't know Hildur Guðnadóttir and I'm glad to see a woman as score composer of a major production. Music is about interpretation and we definitely need to hear more about how women “see” music! Of course Hildur wasn't chosen for the role of score composer just because a woman, but because she is a skilled cellist. The cello is the perfect instrument to describe the tormented nature of this ultra-realistic Joker. The cello is soft but sharp, melancholic and apparently weak, but it adapts well even when more epic times come like in the last instrumental track of the movie: Call Me Joker, which accompanies the liberation of the Joker and his coronation by the rioters.

But the Joker soundtrack doesn't end with instrumental music. You will soon dip yourself in the best pop/rock music appeared in the 20th century. It's always an artistic orgasm to hear legendary songs fitting perfectly in the scene they're put in. I love the sequence featuring White Room by The Creams, where the Joker is brought in the police car. Gotham is literally on fire under the rioters' fury and the Joker is ecstatic in “watching the world burn”. The song talks about a white room (Arkham where Arthur is going to be brought to) but it also says: “You said no strings could secure you at the station.”
Another notable (and noble) mention is my favorite Frank Sinatra song: That's Life was such a surprise for me to find it here in this film! I recognized it already when it appears as instrumental version in the scene where Arthur walks in the club to deliver his first stand-up comedy!
I look forward to bringing That's Life to the Russian Karaoke!

Did this happen for real?

There are a couple of scenes that make you wonder if Joker is a linear storytelling or if instead we are simply peering inside Arthur's imagination.
Almost at the beginning, Arthur talks with his state psychiatrist, when she asks him: “Do you know why you were interned?”. This tells us that Arthur already spent some time at Arkham, which in this universe is a state hospital, not a prison for supercriminals. However, when Arthur goes to Arkham to check his mother's file, it doesn't seem like he was there ever before, unless for another strange scene... For a few seconds we see Arthur standing inside an electroshock chamber. Why should Arthur stand there? Is the electrocuted patient Arthur himself in reality? Then again at the ending... After witnessing the “coronation” of Arthur as new king of Gotham, we are brought back to Arkham again. Arthur is sitting in front of a psychiatrist, looking very similar to the woman seen at the beginning. She just wears a fancier suit and has a tidier hairstyle. Even the environment is cleaner and tidier, the room is all white, transmitting a sense of peace, opposite to the desperate darkness seen in the office of the first psychiatrist. Nevertheless, the outcome is the same. Neither this therapist seems to understand Arthur. Even this therapist keeps asking the same questions. The doubt that this woman in reality is the same therapist seen 2 hours before is legit, and this would mean that Arthur never left Arkham during this whole time! The whole story could be Arthur's own delirium!

Possible sequel idea

Joker is meant to be a stand alone movie. The intention was made clear by the director Todd Phillips. However, I even know that today's Hollywood industry cannot resist from spitting out sequels, especially if the movie was such a critical and box office success as Joker. It seems like a sequel is inevitable. If sequel must be though, it must be different, surprising. It must start from another question arising from the ending. Assuming that Joker story happened for real and wasn't just Arthur's imagination, what would have happened if Bruce Wayne never became the Batman? We could build a story where Bruce is more similar to Thomas, a Bruce who shares some of his father's arrogance, who seeks revenge toward the lower class people, responsible for his family's demise. We could build a Bruce who becomes a ruthless businessman, who uses his financial and political influence to control Gotham as a way to fill the void left in his life. A Bruce with the cynicism and megalomania of Lex Luthor and with a spoon of Joker madness... That would be as interesting as challenging for the whole DC universe!

Conclusion

The power of Joker is that you will end up to sympathize with the super-criminal. In the past we were used to whether hate the Joker or being fascinated by his crazy charisma, but here you will sympathize, which is a strong word. It means that you will understand his actions, you might even justify his crimes and the scariest part is that you would probably act the same way if you would have gone through a bitter life like his.
After all, our society is filled with many mini-jokers. Normally, they don't have weapons or enough madness to hurt physically the others, but every forgotten has the weapon of social media to vent his/her resentment over the successful people and hurt them mentally through hate and harassment. Don't get me wrong, I don't think social media are the cause of this apparent spread of hate that we witness today. I deem social media more as a psychiatrist who detected a disorder in the mind of our society and revealed it to the world. Less fortunate people have always resented the folks who are richer, healthier and more successful, while the latter always felt disdain or uneasiness toward the first ones. Joker is a movie who reminded me that the world needs more kindness and more empathy!
Soon you will forget this article, but don't forget to be more emphatic. Don't be as crazy as Arthur Fleck but don't be an asshole like Thomas Wayne either... Thank you!