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 clownpuppet and the carnivoroustree 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 white, spider-like...thing◊ guarding the door to kids' room when the ghosts return. Somehow the bright light on it (as opposed to darkness and shadows) makes it worse.
And the simple fact that it all unfolds in somebody's tract home in a seemingly ordinary suburban neighborhood, 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?
Bonus Adult Fear: being able to hear your terrified child screaming for you to save her...with no way of knowing where she is.
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 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.
The infamous scene where Marty hallucinates peeling his face off until he can see his skull.
The scene with the meat on the counter moving by itself and imploding blood, and Marty sees it and drops the chicken out of his mouth and maggots are crawling all over it, yuck!
In this version, 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.
The scene with Boyd, when he drills into the wall and accidentally makes a larger hole than expected, and his power-drill falls through. He sticks his hand in, and his arm is held fast by something, which effectively pins him to the wall. Next, his power-drill from the other side is drilling through the wall, with the bit extending several inches into the real world., right at the level of Boyd's head. The drilling comes closer, closer, and Boyd is well aware that the next drilling will kill him gruesomely, but is unable to move. Fortunately, he is released, and he gets away. (It turns out that the scene is probably a hallucination like the face-peeling from the original, as the wall is next shown to be intact.) In the scene immediately preceding this, Boyd set up a death flag by making comments to Griffin indicating that he thought the whole thing was a money-making publicity stunt by an unemployed father; consequently, Viewers had good reason to expect that the above scene would result in a Karmic Death.