Those little doors are way more terrifying than any monster in 'Coraline'. The horror doesn't come from them being open initially, but from the fact they're just there, all the time, in this familiar house. They're sealed shut with bricks at first, which creates this quiet, unsettling curiosity. You notice them like you notice a weird stain on the ceiling. Then, when one of them just... isn't bricked up anymore, the violation feels personal. It's not a jump scare; it's the quiet realization that the boundary between your safe space and something else has been removed without your permission. The horror is in the waiting, in the fact that the door is now an option. That's what gets under your skin.
Also, the sheer mundanity of a door, something you use every day, being twisted into a threshold for the Other Mother's world is genius. The worst part is imagining yourself, as a kid, being drawn to it. The allure is part of the trap. The atmosphere isn't just built on spooky imagery, but on corrupting the everyday. The story takes the most boring architectural feature and turns it into a literal portal to dread.