One of the things that sets Halloween apart from many of its horror compatriots is that its antagonist, Michael Myers, is simply a man.
A man that can take six bullets and walk away, yes; but a man never-the-less.
There’s a scene leading into the climax that I think really illustrates this point. A lot of horror relies on shock moments. The protagonist is creeping along some empty house, she knows the killer is somewhere near by, and then, boo! The killer jumps out and scares her and the audience. Halloween has a few of those moments, to be sure, but the more effective ones are those that show the killer as a person. The scene I referred to was when Laurie (Jamie Lee Curtis) runs back to the house she is babysitting in, and then looks back to see Michael Myers following her on foot.
The fact that she can see him coming, but can do nothing to stop him, makes him a far more imposing and threatening figure than he would have been if he had just disappeared from sight and popped up beside her on the porch. Because Michael isn’t a mythical monster, he’s a man that could show up anywhere.
Even his back story plays into this concept. He wasn’t possessed by a demon or exposed to toxic waste; he wasn’t even abused as a child (don’t believe the Rob Zombie remake). Michael was just a kid that came out wrong. He’s dead inside; as personified by the blank expression on the Captain Kirk mask he wears throughout the film. There’s no reason for his evil, and that makes him unpredictable.
The violence is non-graphic, just a lot of stabbing towards a body and bloodied clothing left in the aftermath. Halloween puts the emphasis on the anticipation of violence, rather than the act itself. The villain is genuinely threatening, so the horror works on an emotional level, and not the visceral one which gorier movies rely on.
Halloween re-invented the slasher film for the modern age; consequently, you can look back and see that most of what it did has become cliche; however, few if any films have managed to find the perfect balance in the formula that this one did.