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 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.
Cry for the Devil: Not minutes after Woo-jin destroys Dae-su's life, the film cuts to a flashback of Woo-jin unsuccessfully trying to prevent Soo-ah's suicide, which occurred because of Dae-su's carelessness. To finally rub it in, present-day Woo-jin promptly kills himself.
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.