This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Poltergeist
There is a reason why some kids are terrified of clowns.
If you're a kid, the scenes with the evil clown puppet and the carnivorous tree are straight out of Hell. If you're a parent, the mother's struggles to reach her endangered children, as corpses pop up out of the pool and floors all around her, are even worse. And the bit where the researcher claws at his own disintegrating face in the mirror gets to everyone.
What about when the giant flaming skull pops out of the portal in the closet? Far more what it represented. Steven was told not to pull on the rope by Tangina - he did it anyway, and what came out attached to the rope wasn't his wife — it was the beast.
And the simple fact that it all unfolds in somebody's home, not out in the woods or whatever, is pretty creepy. How many people who watch this for the first time can avoid looking suspiciously at their closet doors for several nights?
The chairs stacking themselves offscreen. So simple. So threatening.
And done all in one shot, while the camera pans away for a few seconds. Impressive.
Also, look at the bottom-left chair. Two of its legs are resting on air. The chairs are not just stacked, there is a force that is still actively holding them together.
The movie has an uncanny tendency to have long scenes of relative calm and normalcy (even after the poltergeists have made their presence known) only for the HSQ to suddenly turn up out of nowhere as things start bursting out the closet, toys start moving around on their own, and bright lights start shining. It has a thoroughly unnerving effect.
Spielberg/Hooper has several such quiet scenes that give the Genre Savvy audience plenty of time to anticipate something horrible happening. Diane knocking on the bedroom room. Robby hearing the clown hit the floor while Diane is taking a bath. The meter recording spirit activity activates and goes off the scale while the technician is listening to music and drawing in a sketchpad.
A bit of Enforced Method Acting, said to have been Spielberg's idea. Some of the corpses in the final scene were real.
Not some of them were real. All of them were, as using real skeletons was cheaper than getting plastic ones.
Don't forget the very, very end of the film, where the theme music stops and turns into creepy children's laughter, which continues even after the title fades. Not horrifying, but it's definitely unsettling.
Tangina's expository monologue, detailing exactly what was going on in the house and what they were up against.
In the remake, the other side is portrayed as a mirror image of our world, but everything is composed of writhing masses of lost souls, clawing and moaning at nothing and everything.