IMHO the most crucial element of a short story is character. Every genre of short story must have at least one character. The character has a problem. This can be a very small problem, such as a pimple or an exploding water balloon, or a very large problem such as an exploding universe. Then, the story is the character acting to solve the problem (or refusing the call).
There's a lot of debate about whether a character has to be sympathetic or not. I say, nope, the character doesn't have to be sympathetic. But the reader must be able to empathize with him/her/it. The reader must be able to put themselves in the character's shoes (or whatever). This is because a story is really a collaboration between the author and the reader. And, in general, the goal of a story is to evoke an emotional response in the reader. How do we engage the reader? By creating a realistic character.
Stories that make it into our hold-for-voting pile have characters that grab our attention. This can be achieved by a unique perspective/way of thinking, unique experiences, an especially dire or relateable problem, or insert-your-neat-idea-here. There are as many ways to create an effective character as there are writers. When I finish a story, if I can answer the question 'Who was the character and what do I think of him/her/it?' it's a very good sign. If not...