That's a loaded question, to be sure.
First, editors want what they want. No matter how good your story is, if it happens to be a genre or style or narrative voice or POV that the editor isn't particularly interested in, you're unlikely to sell it.
Second, editors definitely want stories that actually fit their magazine. Seems like a no-brainer. But there are too many writers who send Sword and Sorcery stories to Mainstream fiction markets, or Mystery writers who send to Western fiction markets. Research your markets. Make sure you follow that market's guidelines for submission.
Third, editors want to be absorbed into the story as quickly as possible. If you haven't heard of the Lucky Thirteen, I'm going to tell you about it:
The idea is that, if properly formatted for submission, a short story contains exactly thirteen lines of text on the first page. In that thirteen lines should be some element that's going to get the editor to turn the page--something to interest him, something to make him think, "Hm. I wonder what's going to happen next." That's called a hook. Hooks don't need to be overt. They can be subtle. But they should exist. That's why they say never to open a story with a weather report. How uninteresting is that. I know of some editors who will instantly reject if the story opens with weather. Period. They don't have time for the inevitable untrained author's reply of, "But, just wait until you see what happens on page 2!" If page one doesn't at least interest the editor, I guarantee he's not going to turn to page two.
Editors want to see characters they can relate to.
Editors DON'T want to see surprise endings. Really? Yes, really. But "Of course!" endings are superb! What's the difference? An "Of course!" ending means that you have properly foreshadowed the events in the resolution of the conflict. It's the concept in a well-written murder mystery in which the author successfully keeps the identity of the murderer only guessed at until the end. If he ends and the revealed murderer is too obvious, it's a let-down. If he ends and the the revealed murderer is someone that the reader never even suspected, or that the reader couldn't conceive of as the murderer. Success comes when the reader and the author put all the pieces together at the same moment, and the reader says, "Oh! Of course! Cool!"
If the ending exposes an element that has little or no foreshadowed setup through the story, that's a let-down as well. That's a surprise ending, and they don't work. As an editor, I hate them. I hate when the author invests me in a story, then throws something at me in the last parpagraph and I think, "Where did THAT come from?" Like a horror/adventure story in which it is revealed in the very last paragraph that the Main Character is actually a vampire! Oh, my! Doesn't work!
Editors DON'T want to see poor grammar and spelling.
Editors want to see that you're smart and you've done your homework. They want you to demonstrate to them that you have looked over their submission guidelines carefully. They also want you to demonstrate to them--through your writing--that you are not just some hack with a computer and a dream, but a serious student of the craft of writing.
For a good book on what editors do/don't want to see, try: Self-editing for Fiction Writers by Browne and King.