Writing a short story is a different experience from creating a longer one. This seems like an obvious statement, but you don't realize how true it is until you've attempted both. I feel that the biggest difference is in how you pull the reader into your world.
In a longer story, the characters are introduced slower. A reader will come to know your characters' personalities through dialog and bond with them based on how they react to situations. This type of relationship takes time to develop.
In a short story, the reader is instantly drawn in to a world already in motion. The bond a reader feels has less to do with the dialog and more to do with emotions. The character is worried and therefore, you feel worried. The trick when writing short stories is for the author to make the reader care about the characters in a very short amount of time.
Just as a reader is pulled into a world already in motion, so too do they leave it that way. A short story does not have to end the same way a longer one does. This is one of my favorite aspects of this genre. In a longer story, a reader will cry out in protest if too many loose ends are present. You feel gypped. All that time invested in reading the book and nothing is resolved.
A short story, on the other hand, may end at any time. The uncertainty of the character's fate is half of the fun. Since the reader came into a world already in motion, it is up to the reader to decide how future events will fall into place. The job of the author is to leave just enough clues to enable that train of thought.