Psycho/YMMV

Adaptation Displacement: Who remembers the novel the film was based on?
- Bear in mind that Bloch's novel was published in 1959, and the movie was released a year later (Hitchcock gained rights for it almost immediately after it was published); the novel hadn't been out long enough for people to even know of it. Also, Hitchcock went out of his way to prevent the audience from knowing the ending by buying mutltitudes of copies out from his pocket.
Award Snub: Alfred Hitchcock is the most egregarious example (although for Psycho, he was nominated for Best Director, but did not win), but many are utterly shocked that Anthony Perkins was never nominated for his performance as Norman Bates. In 2009, Entertainment Weekly considered that snub to be the second worst Oscar snub of all time. Hitchcock himself expressed to Perkins how ashamed he was because Perkins was not nominated. Also, Bernard Hermann's score wasn't nominated either.
Awesome Music: Composed, once again, by Bernard Herrmann.
- The famous repeated minor-9th violin chords during the shower scene were so monstrously effective, they were used again by later generations of horror movie directors in their own films. They even have their own trope page.
Non Sequitur Scene: The 1998 remake has random, near-subliminal video clips (storm clouds, a sheep in a road, a woman in bondage gear) which pop up during the two murder scenes.
Freud Was Right: Norman's Mommy Issues and sexual repression are probably what drove him insane. Then there's the shower scene, where the knife stabbing is often interpreted as a phallic symbol.
- Another example, a personal one for Norman: at one point he's clearly stuttering and hesitating to say "fallacious", because it sounds like "phallus". He eventually settles on saying "falsity".
Genius Bonus: All the paintings in the parlor, but most specifically the painting that Norman takes off the wall in order to spy on Marion, are versions of Susannah and the Elders, a biblical story about two lecherous voyeurs who try to take advantage of an unsuspecting young woman while she's bathing. Also, in part with the bird motifs, Norman accidentally knocks off one of the pictures of birds in Marion's room, which confirms him as the murderer because he "offed the bird" - which, in British slang, is killing a young woman.
Hilarious in Hindsight: Psycho was released six years after Hitchcock's Rear Window, where the character of Stella questions where Thorwald, the suspected murderer, would've killed his wife in their apartment. "Of course, the bathtub! It's the only place he could've washed the blood!"
Jerkass Woobie: Lila in the second movie. She's motivated by Norman murdering her sister, so you can understand her personal grief. However, she is downright vindictive towards a person that paid his debt to society and trying to move on with his life, so she is not that sympathetic.
Memetic Mutation: The Psycho Strings are the most famous example, but two of Norman's lines are fairly common Shout Outs in other genre pieces:

We all go a little mad sometimes.
Well, a- a boy's best friend is his mother.

Narm: "Mother, oh God, mother...blood! Blood!" Mostly because they are the only piece of dialogue in a lengthy segment with no dialogue at all and thus seem out of place
Pragmatic Adaptation: The film is very different from the novel: most notably, Marion Crane only appears in one chapter, and the shower scene takes place offscreen.
Rewatch Bonus: After you know the ending (assuming Popcultural Osmosis hasn't affected you too much), you'll watch it again and kick yourself for failing to notice all of the Foreshadowing.
Sequelitis: Averted with the surprisingly-good Psycho II. YMMV on the other sequels, but the common consensus seems to be that Psycho III is visually interesting but flawed and lacking in depth, whereas Psycho IV is just plain lousy, albeit not completely awful. The remake and Bates Motel, of course, do not count.
- This also occurred with an entirely different sequel to the original novel, and another plot was bandied about elsewhere: with Norman being released, found to be cured, back into the general populace. Except it was the 70s, when he went away in the early 60s. The eponymous "psycho" would have been the Anvilicious world around him.
Star-Derailing Role: Subverted yet played straight: History will tell you that Psycho was not a box-office nor a critical failure, and it's obviously considered one of the best films ever made. However, many consider it to be the film that simultaneously heightened and ruined Anthony Perkins's career as an actor because he was subject to typecasting afterwards, and most moviegoers only know him for Psycho. Perkins never had any backlash because of it, but plenty of his fans will vouch Perkins was one of the most underrated actors in Hollywood history, and Psycho is to blame.
- Ironically enough, Psycho was Perkins Playing Against Type. After Psycho, if there was a movie where he wasn't playing a "psycho," it was him playing against type again.
- The same applies to Janet Leigh. Although not subject to typecasting afterwards, all she is really remembered for was the shower scene.
Tear Jerker: To people who grew to like and/or relate to Marion while considering her the main character of the movie, the sight of her lifeless body lying on the bathroom floor after the shower scene. It also looks almost like she sheds a Single Tear (or is it just water dripping down her face?) right before she completely dies.
The Woobie: Norman. He's so epically messed up. His father dies when he is five (it's never made entirely clear how this happened). After this, his mother deliberately isolates and dominates him, making him totally dependent on her. She also fills his head with how sinful and evil women are. Then she shows up with a man when Norman is age 12. At this point, there are two possibilities: Norman is angry and jealous and kills both mother and her lover. Or, Mrs Bates kills both herself and her lover (remember the deceased Mr Bates). For option A, Norman is wracked with guilt and retrieves his mother's corpse as well as internalising her in order to alleviate his remorse. For option B, Norman simply assumes guilt for his mother's death (easier than confronting her abandonment of him) - and then internalises her. Either way, the woman who was responsible for his abusive childhood now lives inside his head, berating, watching, punishing - no peace, nowhere to hide. Norman's trap is his own mind, and he can't escape. Hitchcock deliberately cast Anthony Perkins in the role to emphasise Norman's woobie quality. The sequels trade heavily on it, too - especially Psycho II.
- You may feel even worse for him in the second movie, as you know all of this going in. We see that he served his time and met the requirements to be released. He so desperately tries to get his life in order and maintain his newfound sanity, but he's being pushed over the edge due to a personal vendetta.
What the Hell, Casting Agency?: Vince Vaughn as Norman Bates in the remake. Also, taking the Janet Leigh part of the sexy, tempting woman who awakens "Mother" - bony, spiky-haired Anne Heche. Roger Ebert went so far to say that William H. Macy was the only actor who was appropriately cast.