One major issue is you must connect with the reader on an emotional level. To scare the reader, you have to make the reader care about the characters. Thus, usually, the opening of such a story should make the reader empathize with the point-of-view character. (In a flash piece this can be very difficult.) I think this is especially important in horror fiction if you want to scare the reader, make them uneasy. In such cases, first, you have to make them easy. :)
Another issue is there is a long, rich history of horror fiction in our culture. Please be aware of it. If an author expects a surprise reveal that the protagonist is actually a vampire/ghoul/ghost/werewolf/axe-murderer to work, think again.
All that being said, we enjoy macabre fiction. Show us something fresh!
Early Happy Halloween!