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.
Alternate Character Interpretation: Is the child at the end of the fourth movie Kayako reincarnated, or Kyoko's original offspring possessed by the former while still in the womb?
Complete Monster: Takeo, even before he became part of the curse (but only after his Freak Out). He murders his wife (by not only snapping her neck, but leaving her still alive, paralysed and in sheer agony for a period of time before he finally kills her with a knife), his child, and his child's cat in a fit of extreme jealousy - mostly looking extremely calm (if detached) while he does so - and then brutally murders Manami, Kobayashi's wife, in her own home, cuts out her unborn child, sticks it in a plastic sack, phones Kobayashi to inform him of what he's just done, and then proceeds to bash the foetus against railings in the street. After his death, his other assorted atrocities have included him driving Kyoko completely insane by forcing her to watch what he did to Manami, possessing Tatsuya, and murdering Rika (in the novel, he tortures and rapes her instead).
Hell Is That Noise: Kayako's death rattle. Enough said. Also, the sound of a cat's meow has never sounded as creepy as it does in this series.
The sound of Takeo's breathing, although barely audible and only heard for a few seconds, is utterly creepy.
An example from the fourth movie: don't think the sound of a newborn baby crying is scary? Think again.
"Okasan! Okasan! Okasan!"
That absolutely horrendous moan that Megumi emits after becoming a ghost.
The sound of the phone ringing in 4444444444 and Mizuho's vignette.
This trope is even featured on the official soundtrack for the first four films, Ju-on & Ju-on 2 ~The Series Perfect Edition~. The very last track on the CD? A 42-second sound clip of Kayako's death rattle, starting off as fairly quiet but steadily getting louder and louder. It... isn't advisable to listen to it with your headphones in and with the volume cranked up. The track is entitled The Moaning That Was Crushed, which is a pretty good indication of just what the track consists of.
You can even download Kayako's death rattle as a ringtone from the official Ju-on game website.
Moral Event Horizon: Takeo crosses this when he murders both his wife, son, and son's cat in a jealous, paranoid rage. Not only that, but he also murdered Masami, the wife of Kobayashi, the man Takeo believed Kayako to be having an affair with, before cutting out her unborn child, shoving it in a sack and beating it against the walls and floor, and then telephoning Kobayashi to tell him what he'd done. Yikes.
Nightmare Retardant: Some view the fourth movie's "wig scene" as this, however, most agree that the climax of that scene is utterly, utterly terrifying (it is one of Kayako's more frightening appearances to date).
The scene with Toshio's head on the floor in the fourth film.
Kayako pursuing Nobuyuki in the second film. The fact that her crawling in this scene is in stark contrast with her usual Marionette Motion is a big factor.
Paranoia Fuel: Kayako or Toshio can appear from any place at any time. Even from the most inconceivable of places such as body parts!
Sequelitis: The second TV movie is often looked down upon, although it does have its share of genuinely creepy moments. The two movies that came afterwards were much more well-received.
Squick: Several moments - the mutilated, sticky remains of Toshio's cat Mar in the first movie, the remains of the rabbits in the Katasumi short and Kanna's unfortunate demise are all notable examples, however, the moments that really turn up the squick factor are the scenes with Masami's foetus in the first movie, and the birth scene at the end of the fourth movie.