What makes for..

Keeping the evil unknown. Many times the monster loses the scare factor because it’s hard to make them actually scary.

There will be exceptions of course, but even when you know who or what the monster is, the scariest part of them might be the mask. Halloween, Texas Chainsaw Massacre, etc.

Plays on the human psyche. Stories of monsters and boogeymen are just jurrasic park with a few added jump scares. Stories of evil spirits, possessions, and hauntings...things that cannot be seen outwardly...those things are what really scare us. We're scared of what we cant see, what we cant understand.

Stuff crashing for no reason and have a loud impact

Best horror series I've seen in awhile that and "The Terror"
I agree with Boma about "keeping the evil unknown" what we conjure up in our imaginations is far scarier than anything we can physically see. Also unnecessary blood and gore makes horror movies more comical than scary. Jump scares are cheap and there is nothing scary about a ghosts opening and closing doors, banging cupboards, breaking windows etc.

I started to watch the series but my husband wasn't too keen on it. I will dare guess it isn't half as scary as the novel though, as I have only seen one film that came close to the horror of the material it was based on. But The Haunting of Hill House indeed scared me more than any other work of fiction.. I think this is because of all the psychological tinkering Jackson did with her heroine via the interrelationship with the paranormal stuff going on around her. By the end of it I knew her entire story and had run through a gamut of emotions for and with her -sympathy, hatred, shame, desire, terror and abject loneliness. Very disturbing, absolutely unforgettable.
Like you, blood and gore is not scary to me. Just annoying.

Just ordered the book. Sounds like my cup of tea :)

Cool! :)