Honestly, I've always found the ghost-as-mirror thing way more unsettling than jump scares. It's not just about creepy visuals; it's that slow, dawning horror when you realize the spirit is reflecting something the protagonist refuses to acknowledge. Like, that ghost in 'The Haunting of Hill House' – isn't it partly Eleanor's own desperate, lonely self, twisted into a supernatural form? The suspense builds because the character can't run from an external monster, they're being forced to confront the internal one.
Another layer is the violation of rules. We live in a world governed by physics and logic. Ghosts operate on their own, often contradictory, lore – they might only appear at a specific time, or through a specific medium, or their power might be tied to a forgotten memory. The suspense comes from watching characters try to piece together this insane, shifting puzzle while the spirit's influence grows. It feels like a slow gas leak; you know something's wrong in the air, but you can't see it until it's too late. That's the real gut-punch for me, that sense of a reality coming unglued.