The musical score is both delightful and absolutely bone-chilling.
The moment when you briefly see Myers standing in the street in the broad daylight, before he steps behind a hedge. It really sets up the predator/prey dynamic between him and Laurie. She is being watched...... Probably even more scary because some real-life serial killers claim to operate in this way.
The scene when Laurie finds the bodies of all of her friends throughout the house. And then you see Myers slowly materialize right behind Laurie in the closet....
Word of God makes it even scarier. It's not Michael walking into the light, it's simulating your eyes adjusting to darkness. Michael was standing there the whole time, he just wasn't visible in the dark.
The fact that he's got no reason to kill. While serial killers generally have something motivating their kills, Myers has been messed up since childhood. He doesn't even kill For the Evulz. By all rights, he should be a Generic Doomsday Villain (and he kind of is). Instead, it's extremely unsettling and pure nightmare fuel.
Fun Fact: Michael Myers was inspired by a thirteen year old boy John Carpenter saw in a mental institute he went to on a school trip.
The shot after Michael had killed Bob. Michael just looks at the corpse and studies it, tilting his head to one side. What is going through Michael's head?.
Laurie is able to grab the knife away from Michael and stab him, and he falls to one side. Laurie tells the kids to run out of the house and get help, and then just rests there in the doorway, thinking it's all over. And then Michael slowly sits up and turns his masked head at her...
That is really sold by the score, which is nightmare fuel in itself in some ways. Just how perfectly timed the dun - dun-dun! fits the action is stunning, but then that is a Carpenter trademark.
The way Michael is integrated into so many shots. There's one scene where Annie is in the kitchen talking on the phone. Directly behind her is a pair of glass doors leading outside. For a brief moment Michael can be seen standing there watching her through the glass.
An injured and terrified Laurie runs to a neighbor's house, bangs on the door, and screams for help. Someone inside turns on the lights, goes to the window... and then drops the blinds and turns the lights off, completely ignoring her. Realistic? Unfortunately... yes. Therefore all the more terrifying.
But remember, it was Halloween. In the Novelization it's made very obvious that her screams for help are seen as just another Halloween prank.
Michael wearing the bedsheet. The image Michael appearing in the doorway is bad enough. Then you factor in that he just killed a guy and put his glasses on the ghost in order to trick his girlfriend so he can kill her without her even realizing a killer is in the room. Probably the first genuinely frightening example of a Bedsheet Ghost.