The theme music. Quietly chilling and suspenseful, just waiting for a slasher to appear.
The fact that Michael Myers isn't just some psycho-killer wearing a mask. The mask shows his true side, a soulless Shape who forsake his humanity, and who kills anyone in its path.
And unlike his fellow slasher villains who either became more AffablyEvil or goofy as time went on, Myers stayed the same. He stayed the same evil force from the first film to the last.
Well, until the Zombie films, anyway.
Hell it took the SECOND movie for Michael to become silly as the first one showed him as an almost even WORSE force of nature than the first one with a higher body count.
There's also one fact: The times Michael is shown without his mask..he's just an average person. And that's what so terrifying too and Carpenter's reasoning behind Michael. There isn't a reason Michael Myers is a killer. He just does it and that's it. And it leaves a lot to the imagination.
The Soundtrack Dissonance. The damn soundtrack dissonance! Especially the way Halloween H20 uses it to call back to Halloween II. This series will ruin Mr. Sandman for you.