Awesome Music: "Three six nine, the goose drank wine, the monkey chewed tobacco on a streetcar line...", a catchy chant done by kids playing jump rope in Dave's fantasy.
The score, by James Newton Howard, is dark and gothic, with religious overtones and heavy use of choirs. The closing theme is beautiful.
Big-Lipped Alligator Moment: Dave rappelling out of his apartment after he's put on probation. It's random and doesn't really have an explanation, apart to set him up as rebellious, which doesn't come into play in the film (beyond his willingness to participate in the experiment at all).
Designated Villain: Only Nelson really seems to have a legitimate reason for his personal hell, as his bullying resulted in another child's death. David verbally teased a girl when he was about eight years old, Joe cheats on his fiancee and records these encounters without his partners knowing, and Rachel witnessed her father shooting heroin, which led to his suicide. The first is pretty weak if that's the worst thing he's done in his life, the second is sleazy and unethical but not exactly high grade evil, and the third wasn't even her fault! The Unfortunate Implications is that if you ever did anything in your life that either had malignant intent OR caused suffering (even unintentionally), you'll be suffering your own personal hell after you die.
One interpretation that lends weight to the "self-induced hallucinations" theory is that the visions are driven not by the objective degree of evil the "flatliners" have committed but by their unadmitted guilt over their past actions, a guilt that can only be released when they have managed to come to terms with it (whether by undergoing an appropriate form of penance, or by finally understanding that it was not their fault to begin with).
Ho Yay: Virtually every scene where Nelson and Dave interact. The two are huge Foils for each other, making for a quasi 'opposites attract' dynamic.
Narm: There is something ungodly hilarious about a ten-year-old girl appearing out of nowhere on the subway to hurl a volley of swear-filled-insults at a bemused Kevin Bacon.
Nightmare Fuel: Billy Mahony's apparition is terrifying. He's a Creepy Child who appears at intervals to attack Nelson. The flashback to his death provides plenty more of this on its own.
Tear Jerker: Rachel confronting the ghost of her dead father. She had spent her whole life believing that she somehow was responsible for his suicide. Here she learns that it isn't her fault and she finally forgives her father.
