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.
Rust: Is she a malevolent alter-ego of Sabitsuki created by a disease to take over Sabitsuki's body? Or is she really an alter-ego created by Sabitsuki to defend herself against the virus and other things that could possibly threaten Sabitsuki? Is she a protective split-personality turned malevolent and trying to kill Sabitsuki? There are all sorts of interpretations of her one could take away.
Smile: Sabitsuki's brother, former boyfriend, former platonic friend? A popular boy in school that Sabitsuki looked up to and/or felt overshadowed by? Did he intentionally hurt Sabitsuki at one point (Corrupted School event), and was it either out of malice or self-defense? Is he an Author Avatar of lol or simply just another character?
Sister: Just Smile's shy little sibling, or an innocent-looking girl with a secret dark side? The latter interpretation seems to be fueled by lol's artwork of a Sister-like girl.
The Cleaners: Are they simply maids that work at a bar, or is there something more shady behind them? Are they connected to the asylum the Sugar Hole is located next to, or the seedy Psychedelic Town, or both? And what about their appearances in the endings?
Ambiguous Gender: The short-haired Kaibutsu models have been interpreted as both male and female, due to the ambiguity of their sprites. However, the twintail model is always interpreted as female, and the Sabitsuki model is undebatably female.
Big Lipped Alligator Moment: The "gore demo", which is a strange event of flashing NPCs and pictures over a black screen while a slow heartbeat plays. It is activated by getting all 24 effects and using your computer, and you're booted off the computer after. Some fans take it as a "preview" by the game to warn that the game is going to become much darker (hence the name, "gore demo"), but it's still weird.
Demonic Spiders: The Kaibutsu, which are pale white-haired figures resembling schoolchildren with bleeding faces. Usually they simply wander around, minding their own business, but then if you smack them with the iron pipe, they become enraged and chase you. Touching them teleports you to inescapable areas. Some of them don't need to be smacked to be enraged, since they already are. Luckily, you can avoid them by using the Ghost effect's invisibility cloak, but touching them still teleports you away.
Crack Ship: Sabitsuki is sometimes shipped with the Kaibutsu.
Ear Worm: Many of the songs, scary or not, but especially Shinsoku Neko's music.
Ensemble Darkhorse: Sister notably gets a rather disproportionate amount of attention in the fandom, for an NPC that only has a minor appearance in one place, where she is notably overshadowed by her brother Smile. lol drawing lots of art of a Sister-like girl probably helps.
Even Better Sequel: Some view .flow as the superior game to either or both of Yume Nikki and Yume 2kki, usually due to .flow have a more tightly-packed and coherent theme and world compared to the more random nature of the other two games, and having some semblance of a plot.
Goddamn Bats: The red demons. Though they only appear in one part of the game — specifically, the deeper areas of the Womb Level with floating mouths in the background — they nastily disable your effects upon contact and add a little to the corruption meter, not to mention there are a lot in said areas. In the endgame, though, they become even worse. They're just about everywhere at this point in the game, they slowly wear Rust down, move erratically, and have an annoying habit of speeding up when they have a clear path to Rust. On the other hand, they make certain spots that normally had Kaibutsu easier as they replace a lot of them, meaning Rust can just tank the damage and move on. Except that if you touch too many, you explode in a bloody mess!
Paranoia Fuel: Although not as abundant as the Nightmare or Nausea Fuel, the game certainly has its paranoia-inspiring moments; for example, the Streets area and its resident randomized event, and the Nightmare event.
Squick: Some theories speculate that the Sugar Hole's cake is made out of human flesh.
The Orange menu is obtained from a ghostly Oreko in an area where melancholic music plays, implying that Oreko died at some point, but still wants to help in anyway she can.
In the Children's Hospital, you can find a younger version of Sabitsuki. She's bedridden and attached to an IV. If you level up the corruption meter, her condition deteriorates and more wounds appear on her body until she looks inhuman.