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.
Crowning Moment of Awesome: The hallway fight. Dae-Su fights a whole bunch of henchmen down a hallway with just a hammer and his bare fists. After clashing to exhaustion, with a knife in his back to make things harder, he leaves them gasping for breath and on the floor. He proceeds to the end of the hallway and more of the henchmen appear from the elevator, but he deals with them without breaking another sweat. BADASS.
For the director as well, given that the scene is nearly 5 minutes long, and is all filmed on one camera, in one take. The fight sequence took three days worth of attempts to nail.
Crowning Moment of Funny: When Oh Dae-Su walk away from the upset woman and the suicide guy falls on the car close by. The smile he gives clinches it.
Crowning Music of Awesome: The first movement of Vivaldi's "Four Seasons: Winter" certainly takes the cake, but the entire soundtrack of this movie is a work of genius. Every track is named after a famous movie classic (most of them film noir) yet every title also clearly applies to the events seen in the film when the track is played. Every major character has a theme that is played overtly in several scenes, rather than recurring only as background music. Lee Woo-Jin's theme is lovingly used throughout the film; it is also heard as the jingle marking the release of the gas in Dae-Su's prison and it is the ringtone on the phone given to him by Woo-Jin. The pieces themselves are so appropriate and original that even hearing a few seconds of any part of the soundtrack will instantly conjure up the atmosphere of the movie.
Esoteric Happy Ending: According to the director, it's either a happy ending that's sad or a sad ending that's happy. Either way, the implication is that the protagonist continues to carry on an incestful relationship with his own unwitting daughter, and that he may or may not know himself.
Jerkass Woobie: Lee Woo-jin. Sure, he is an evil man bent on revenge against Dae-su. But he gets sympathetic by the end of the movie when he watches his sister die.
Paranoia Fuel/Fridge Horror: The man was locked up for 15 years, and tricked into sleeping with his daughter. All because he mentioned something he had seen, once. Can you honestly say that you've never done anything as bad or worse than what he did? I didn't think so.
Squick: Played first for laughs (like the octopus scene), but getting steadily darker as the film goes on. Especially the ending.
Tear Jerker: Several scenes, but especially the unbearable finale when Dae-su screams and grovels before Woo-Jin, begging him not to tell Mi-do that Dae-su is her father.
"The Last Waltz", which plays during Mi-do's fantasy of the ant on the train and during the end credits, is a Tear Jerker all of itself, when the movie is over.
The Woobie: Oh Dae-Su. The dude gets locked up alone in a hotel room for fifteen years straight without being told why, and then finds out that he's been manipulated into banging his own daughter.