These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Award Snub: The film won an Oscar for Best Screenplay, but probably would have won a few more, perhaps even Best Picture, if not for the misfortune of being released in the same year as The Godfather Part II. A lesser example than most, because the winner is also a great film. Chinatown did beat The Godfather Part II for Best Drama at the Golden Globes.
John Huston plays one of the most chilling villains of all time, and yet was not nominated for a supporting Oscar.
Complete Monster: Noah Cross is a 1930's Corrupt Corporate Executive who doesn't even know his own net worth, outside of the fact that it's a great deal over ten million, only caring about accumulating as much money and power as possible. In the midst of a drought in Los Angeles, Cross dumps away thousands of gallons of water in order to build up support for his new reservoir project. Drying up the northwest San Fernando Valley by depriving it of water, Cross also poisons the wells and blows up the water tanks of the valley's landowners to coerce them into selling their land dirt cheap. Once the valley is his, Cross plans to use his new proposed reservoir to irrigate the land and increase its worth by millions. However, his business partner, and son-in-law, Hollis Mulwray, hinders his plan. Previously Cross had convinced Mulwray to build a dam in an area known to be geologically unstable. The result was that the dam burst and 500 people died. Even though Cross knows that the new reservoir would suffer from the exact same geologic instability as the previous one, he still demands Mulwray build it. After Mulwray discovers Cross's crimes and conspiracy, Cross kills him by drowning him in a tide pool. However, the most disgusting crime Cross committed was what he did to his daughter Evelyn. When she was just 15 years old, Cross raped her, traumatizing her for life and fathering another daughter with her. Cross's secondary goal in the movie is to take possession of his "grandchild" with the intention of subjecting her to the same abuse he subjected Evelyn to. Despite all of this, Cross says he doesn't blame himself for his actions because he believes, under the right circumstances, men are capable of anything, ignorant of the fact that this is only true for men like him.
Crowning Moment of Funny: Near the beginning of the film, a bunch of shepherds disrupt a public gathering by unleashing a flock of sheep on those gathered. This actually cracks up Jake Gittes.
Harsher in Hindsight: The sexual abuse Noah Cross commits within the movie could be seen in a whole new light after attention was brought to a real-life court case implicating Polanski himself in something similar. This article on Cracked explicitly notes the parallels between how Cross gets off scott-free for his crimes and how Polanski himself escaped jail time by fleeing the U.S.
An interesting use of this trope. John Huston, as Noah Cross, has all of two (three at a push), very brief, scenes in this very long film. And he's often remembered as one of the most despicable villains in cinematic history.
Also Diane Ladd as "Mrs. Mulwray" (aka Ida Sessions), who hires Gittes in the first scene. She does turn up a couple times later, first as a voice on the phone and then as a corpse.
And Polanski himself, as the Man with the Knife. "You're a very nosy fella, kittycat. You know what happens to nosy fellas? No? Wanna guess?"
Not only that, Noah Cross gets away it with all. Mrs. Mulray is dead, her sister/daughter is taken away by Noah, Jake can't do anything to stop this and is told the famous line before solemnly walking away. If any of that didn't make you jerk a tear then the absolutely depressing music that plays when Jake's walking away certainly will.
Simultaneously subverted the ending leaves the viewer feeling gutted and raw but not weepy, is a heroic BSOD for the viewer