I really, really don't understand this complaint. Yes, it's a horror film, but once you introduce the unrealistic elements, the interactions and what happen according to the rules you introduce, should be realistic and consistent.
Lets have a look at a few of my favourite 'unrealistic horrors' in the fact the deal with things that couldn't happen in real life:
The Exorcist,
The Shining,
The Thing,
The Fly.
They are still realistic in their execution, once you accept the character of the possessed girl, the morphing 'thing' and a man-turned-fly, the films feel very real, their world and other characters react according, and it's this human level, psychological thriller aspect that makes them successful.
Halloween gives the impression that it is like them a character driven horror, that what we are seeing is real, everything seems pretty plausible up until the third act where I think the main characters actions are completely stupid.
I praise the film for its atmosphere and direction, Carpenter uses the camera to achieve this effectively, he's clearly watched enough films to know how to achieve the stylistic look he goes for, but I think in terms of story its quite thin, and once we reach the finale it descends into a bit of a farce.
But yeh, I'm (were?) in the minority here, and I understand a lot of people consider it a classic of the genre, that's fine