It's all about the spectacle:
Eye candy costumes (and ingestible candy...mmmm, caaannnndyyyy, chocolate aaaaahhh)
Scary Decorations (or at least campy ones!)
"Haunted" Rides and attractions
There's something about going out into the dark unknown night traipsing through barely familiar neighborhoods as the chill night air seeps through your costume slowly so that you don't notice until a couple of hours later when you're done trick or treating or having successfully navigated the attraction you went through and you're in the car warming up getting oh so toasty.
But also, and this goes along with the eye candy and ingestible foods, there's the colors, smells and tastes of Fall/Autumn. Reds, oranges, yellows, and browns in the trees and fields, not to mention pumpkins and Jack-O-Lanterns!
And the unsettled weather, whose blustery winds make strange noises in the trees and the rain which turns the streets into a Noir film adding to the sense of general unease.
Foot steps echo down the hall, some lights are on but not them all
The Spirits wait with baited breath, to see who's next upon their death
Within this night of unbridled doom, A creature wakes within a tomb
The casket lid slides to one side, there is not one place to hide
A piercing shrill splits through the night, another person falls to fright
Heed the words of those who know, or else wind up six feet below.
---Mark Osmun 10-18-2010