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. He was standing there the whole time.
The look of resignation and completely unhappy acknowledgement that he was right at the end of the film; his speeches to the police chief aren't hyperbole, he was totally right, and now the monster he's tried his best to keep locked up is now loose.
The fact that he's got no reason to kill. While serial killers generally have something motivating their murders, 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.
When six-year-old Micheal is unmasked, he has this sort of confused look on his face, like even he doesn't know why he did what he did.
Fun Fact: Michael Myers was inspired by a 13-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?
Something similar occurs right after this. Michael starts violently strangling Lynda as she tries to call Laurie. Laurie listens to Lynda crying out in pain, and initially assumes her friends are playing a joke on her before becoming genuinely worried. Once Lynda's dead, Michael picks up the phone and just listens to Laurie as she starts to panic.
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. Michael can be briefly seen 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 Bob and put his glasses on the ghost in order to trick Lynda 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.
The opening scene where young Michael has just murdered his sister and wandered out into the street, just as his parents are coming home, finding their son wearing a creepy clown mask and clutching a bloody kitchen knife...
Mr. Myers: ...Michael?
Every murder in the movie is filmed in a particularly unnerving way. Whether it's the disturbing POV sequence as young Michael stalks and stabs his own sister to death, the incredibly creepy manner in which he kills Annie and Lynda, or the way he easily overpowers Bob, lifts him off the ground and impales him into the wall.
Michael's escape after the film's opening is a brief scene, but no less nightmarish. The mental patients wandering aimlessly outside in the dark, looking eerily like ghosts. Loomis immediately realizing something is wrong and exiting the vehicle to find out what's happening, only for Michael to leap on top of the car and attack Marion when she rolls down the window, causing her to spin her car out of control. The way Michael smashes the car window with his bare hand. And it's pitch black and raining hard during all of this.
Dr. Loomis: He's gone, he's gone from here! The evil is gone!——
Rare heroic example: Laurie stabbing Michael in the eye with a coat-hanger hook during the finale. It can certainly make a viewer wince, especially if they themselves have ever had a similar injury.