A clear plot. In a 500 - word short story, there's no room for complex sub - plots. For example, in 'The Necklace' by Guy de Maupassant, the simple plot of a woman borrowing a necklace, losing it, and then spending years in poverty to replace it drives the story. Also, vivid characters are key. Even with limited space, the characters like Mathilde in 'The Necklace' have distinct personalities. And a strong theme, such as the folly of vanity in 'The Necklace' is essential.
Well, in a 500 - word short story fiction, the first key element is an engaging opening. It has to grab the reader's attention right away. Consider 'The Tell - Tale Heart' by Edgar Allan Poe. The narrator's immediate and frantic tone draws you in. Then, there's the element of pacing. Since it's short, the story has to move quickly. Poe does this by gradually increasing the narrator's paranoia. And of course, a memorable ending. In 'The Tell - Tale Heart', the confession at the end leaves a lasting impression.