Awesome Music: The music for the game is, in short, absolutely stunning. Special mentions go to "My Burden is Light", the title screen music which perfectly captures the atmosphere of the game; and "Self Contained Universe", a peaceful and beautiful track that plays at the end of the game if you choose to put the lightbulb in the spire.
Big-Lipped Alligator Moment: The Secret Penguin Room: it requires you to put a secret code on the countdown door, then you get in a room with melted snow and what seems to be a penguin. The animal doesn't say a word is Niko is confused about what they should do; to add even more confusion, you can actually transform Niko into a penguin for some seconds. And when you decide to leave, a Jump Scare occurs where the room becomes white and the penguin is covered in snow and glitchy squares. After having left the room, Niko suggests to never speak of this again.
Fan Nickname: It's common on Tumblr to refer to the lightbulb as the "sunbulb."
Fridge Brilliance: The game is hard-coded to only be able to be completed ONCE. You're literally given one shot to save the world within the game, the game failing to open completely after you've completed it once.
Funny Moments: The pieces of paper you find scattered throughout the game usually feature puzzle hints or background info for the world, and many of them are as eerie and dark as the rest of this world, until one particular paper that's available late in the Solstice update which simply reads:
WHO TOOK MY STAPLER
Especially hilarious considering that, in the Regular route, the same notes says that the stapler has been glued to the table so that it couldn't be stolen. That's right: In the middle of absolute chaos, when squares kept popping everywhere in the Refuge, someone somehow had the time to steal a glued stapler.
Asking Niko to put the sun in a crusher nets this achievement: Chaotic Evil.
The Inverse Law of Fandom Levity: While not as dark or bleak as other examples on the list, the game is still about a small, defenseless child separated from home who is forced to explore a mysterious, dying world to heal it, with metafictional elements, fourth wall breaking and lots of Tear Jerker moments. The fandom, while aware of all of this, is still very fond of cute fanarts of the characters, fanfics and the like that downplay the drama, and goofy shitposts like theseones.
The setting itself: a slowly decaying, falling-apart world devoid of a sun to light it, and must rely on outside sources to bring back the light again. And the world will continue to die even if the light comes back.
When you cross from the Glen to the Refuge, you have to pass through a corridor first. A corridor where eerie music plays and the area starts decaying behind you as you pass through, with the decay appearing suddenly and accompanied by crackling bursts of static.
Paranoia Fuel: The game does things like adding files to your computer and changing your desktop, not to mention modifying your computer to ensure the game is one shot only, outside of certain situations such as saving/dying, unless you play the game on another computer or restore your computer to before you played the game. While these are relatively harmless acts, it's all too easy to imagine how out of hand they can get. And then there's the general fear that comes from a game modifying your computer in the first place...
The biggest ones occur when starting the game after either killing Nikonote by manually closing the game window in the original game instead of using a bed (the remake instead has Niko comment that "everything went dark" but be otherwise unharmed) or having them return home.
In the Solstice update, in contrast with the first playthrough, where you and Niko are able to care for the residents of the world, the Solstice Route sees many people helping Niko out, and suffering as a result.
Scrappy Mechanic: The whole "one shot" mechanic causes problems if you die by accident (by a power blackout or a BSOD, for example), or finish the game and want to replay it.note The game is not meant to be replayable, but some players dislike this artistic choice. The only ways one can replay the game normally after finishing are to: restore their computer to before the game was played, play the game on a different account, or use a different computer. Seems fine, except one may not have access to any of those options, and at least two of them will become inaccessible anyway after replaying the game using them.
For those who really want to try it again: open "regedit.exe" from the start menu, go to HKEY_CURRENT_USER -> Software -> Oneshot and delete the entry "ending". Strict warning: if you don't feel sure about changing the registry of your PC, don't do that.
A special function was added to the remake which allows a New Game Plus mode, averting this partially, but it requires you to run the game in a special way.
The way to get around this fully in the remake is similar to how to do it with Undertale; turn off Steam cloud saves and then wipe your save directory.
That One Achievement: The 2016 remake turned the Scrappy Mechanic into this. Good news is that you don't have to worry about quitting the game without permanently being locked out of access to the game. The bad news is that if you want the achievement for clearing the game on a single attempt (meaning no exiting the game at any point), you'll have to deal with the accidents as well.
The Woobie: Niko. They're already a lost child separated from their family and stranded in a strange, dying world, but then later on in the game, you find out that they can only return to their home world if they smash the lightbulb, thus rendering the world they're trying to save devoid of a sun up until it dies, and putting the sun back anyway will only end up delaying the inevitable, while Niko will be stuck there forever, will never be able to go back home, and according to the Entity, will always be miserable in the decaying world. Ouch. Thankfully, the Solstice Update allows for a brighter ending for both Niko and the world.
: it requires you to put a secret code on the countdown door, then you get in a room with melted snow and what seems to be a penguin. The animal doesn't say a word is Niko is confused about what they should do; to add even more confusion, you can actually transform Niko into a penguin for some seconds. And when you decide to leave, a Jump Scare occurs where the room becomes white and the penguin is covered in snow and glitchy squares. After having left the room, Niko suggests to never speak of this again.