Undertale/YMMV


 * Alternate Character Interpretation: Hoo boy. Just about everyone invites this to some degree, though the biggest offender is easily the Fallen Child.
 * The reason why
 * And perhaps the biggest one:
 * Awesome Ego: The Great Papyrus!
 * Base Breaker: Alphys and Asgore are easily the biggest examples, with Toriel not too far behind.
 * Asgore has people split into two camps: there are those who view him as irredeemable for being responsible for the deaths of six children and think that Toriel has every right to view him as a despicable monster . Others however absolutely love him for being a rather complex and tragic Anti-Villain who was dealt an absolutely shitty hand in life thanks to
 * Alphys however is even more decisive for similar reasons as Asgore: These have led to people loving and pitying her, while others look at her with scorn and view her as an irredeemably selfish bitch. Not helping matters is how overbearing her geeky, status-update spamming personality is: with some loving her for being a goofy nerd while others loathe her for being obnoxious.
 * While generally well-liked thanks to making a good first-impression, her actions in the True Pacifist ending have really gotten people's opinions split on Toriel. Not helping matters is the relevation that
 * Complete Monster: While they're subjected to a lot of guesswork and theorizing in regards to how evil they truly are, this is definitely how the Fallen Child can come across when one looks at their actions and personality in the game proper. While they were definitely a troubled child in life, a Genocide Run ends with
 * Of course, the player themselves can aim to be one should they wish. While the Fallen Child is the one pulling the strings for the last portion of the game, in a genocide run the player themselves made the choice to kill every monster in their way, no matter how friendly or innocent they were, and you can easily go out of your way to be as cruel as possible: you can stab Toriel in the back after convincing her to stand down, murder the Royal Guards in front of each other after ...
 * While first playing through the game, this is definitely how Flowey comes across: he's a spiteful, hateful, sadisitic little freak who tries to get the player to accept his kill or be killed mindset, murders, and  But as you play through the game more and explore the other endings, it turns out that there's a lot more to Flowey than what meets the eye, and the hidden depths and tragic elements reveal that while he seems to be actively aspiring to become one, he truly doesn't fit the bill.
 * Hatedom/Fan Dumb: during the moments of biggest popularity, the fandom was infamous for generating them, particularly due to contentious aspect as the popularity of Sans bringing a disproportionately high amount of fanworks based on him, the popularity of Alternate Universe Fics (and subsequent works based on those) as the main type of fanwork, the loudness of its ship wars, and controversies on the redeemable potential of its base breaker characters, the pronouns and gender identity of the player character, and so on. Not helping these was that at the time the game had a Friendly Fandom situation with Homestuck, itself prone with similar amount and types of wank. Evidently the fandom has calmed since, but the stigma remains.
 * Most Terrifying Sound: Flowey's laugh quickly becomes one
 * Nightmare Fuel: for such a whimsical game there is a lot of creepy elements.
 * Everything related to Flowey, from his personality, to his laugh, to his ability to change faces (at some point he morphs showing the faces of every main character, including the player), to the reveal of how such a deranged being came into existence.
 * The True Lab. An apparently abandoned laboratory/hospital with clues that experimentation in living beings were practiced in the site, a gloomy atmosphere, Eldritch Abominations roaming freely, and you can't escape the place.
 * The ending of the genocide route.
 * The hints about the existence of W. D. Gaster, particularly if you played the version of the game that activated the random appearance of his followers after having played the version where you has to hack the game to have access to such events.
 * Player Punch: the game doesn't spare a bit into punching you in the feels.
 * That One Boss
 * Mettaton EX can come to this if one is doing a pacifist run, because of the Ratings mechanics.
 * in the genocide route. Specifically stated in universe to be intended to cause Rage Quit, and it shows.
 * That One Level
 * Hotland. In compensation to have one of the best overworld themes of the soundtrack, it brings an insane amount of downright frustrating puzzles, surprisingly stronger monsters, and of course Mettaton.
 * World of Woobie
 * World of Woobie