Horror is one of the most technically demanding genres to execute well, which is part of why the gap between great horror and forgettable horror is so wide. Comedy needs a joke to land. Horror needs an entire atmosphere to hold, and if it slips even briefly, the fear breaks and doesn't easily come back.
Good horror relies on control, controlling what the audience sees, controlling pacing, controlling sound. Some of the most effective scares in the genre's history come from what's implied rather than shown. A closed door, a sound just off-screen, silence held a beat too long. Horror that leans entirely on jump scares tends to age poorly, because a jump scare is a cheap, momentary trick rather than a sustained feeling of dread.
There's also a meaningful difference between horror that scares you and horror that unsettles you. Jump-scare-driven horror produces a quick physical startle. Atmospheric or psychological horror produces something that lingers, a low-level unease that follows you after the movie ends. The genre's most respected entries tend to aim for the second kind, since it's far harder to achieve and much more memorable.
Sound design deserves specific credit here. Horror is arguably more dependent on audio than any other genre, a well-placed silence, a distorted score, or an unnatural ambient noise can do more work than any visual effect. Films that neglect sound design in favor of visual gore tend to feel far less effective, even when the visuals themselves are competent.
Horror also splits cleanly into subgenres that behave very differently: slasher, supernatural, psychological, folk horror, body horror. Each has its own rules for what "works," which is part of why horror fans tend to have strong, specific preferences rather than liking the genre broadly.
On Rolio, your horror catalog is one of the more revealing genres to look back through, since tolerance for gore, pacing, and scare style varies so much person to person. Filtering your own horror ratings however you choose can show you exactly which subgenre you've been gravitating toward lately.