Here are some of the things I see most often that makes a story bad:
Poor punctuation/spelling: Not every sentence needs an ellipse (...) or a semicolon (;). If you don't know how to use them properly, don't use them. Make sure when you end a sentence, it's in the right place; not having a sentence go on forever or ending it too soon.
Flow: The flow of the story can be hard to get. Make sure the story has a beginning, middle, and end. You can start where ever you'd like, but make sure that there is more to go or find a way to make it more interesting. I have a story, Spirit Bottles, that starts at the end and works its way back to that point. (Shameless plug, Trying to get better at that doing those seamlessly.) Anyway, make sure everything moves smoothly. If your characters are running for their lives one second, don't have them eating dinner like nothing happened shortly after. Have it go smoothly from the start to the finish, for the reader, not the characters.
Tension: You need to be able to build good tension for a horror story. Make the reader squirm. You're trying to scare them, so do it. This is something that a lot of stories aren't able to pull off. The best way to make a reader feel the tension is to make them relate to a character. Once they do, throw that character into a horrible situation. That's why Paranormal Activity was so scary. The audience was watching something that could be their normal lives, then some crazy shit happens. The movie pulled them in so well, because it resonated with people. It was life as normal, until the ghost started attacking.
Editing: This is the most important thing that you can do. Find anything that isn't working and get rid of it or fix it. It takes some time to get good at it, and the more you work on it, the better it gets. Put in in the WW to get more eyes on it, and the people here will tell you what can be changed. Don't fight with the people who are trying to help you, because that will just cause people to not want to help. You can learn a lot of things from the people on this site, and it's a great place to get started.