One characteristic is their brevity. They can convey a powerful message or tell an interesting story in a relatively short space. For example, they might focus on a single event or character trait. Another is uniqueness. They often have a special twist or a different perspective. Like in 'An Occurrence at Owl Creek Bridge', the ending has a huge twist that changes the whole perception of the story.
They are often highly imaginative. They can take the reader to strange and wonderful places or situations in just a few pages. And they can be very impactful. A short unique story can stay with you for a long time, making you think about its themes and ideas long after you've read it. For example, Kafka's short stories are so strange and thought - provoking that they've influenced generations of writers.
Well, short unique stories usually have a very distinct mood. It could be eerie like in Poe's short stories, or it could be heartwarming. Also, they tend to be very focused. They don't waste words on unnecessary details. They might use symbols in a very effective way. Take 'The Yellow Wallpaper' for instance. The wallpaper is a powerful symbol that represents the main character's mental state.