1. Multiple typos, unintentional grammar errors, or misspellings. Have someone with a brain proofread your story. 'Nuf said.
2. Failing to reveal the name of the protagonist in 3rd person pov stories. Calling the protagonist "he" or "she" from the get-go isn't dramatic--it's distracting. It's almost as bad as a second person POV story with the main character being "you."
3. Changing POVs within the first few paragraphs. Very few short stories with multiple POVs work. Doing this in the fist few graphs is an even greater challenge.
4. Starting with a "shortcut" for tension, e.g. "It was an ordinary day--until she came along" or "When it happened, I couldn't believe it . . ." [but we don't find out what "it" was until later in the story.]
5. Too much jargon at the beginning, e.g. "When the Yittamaz abandoned the gamboflex, I was sure that we'd lost the Ce-ta-ee forever, but a Soungtelag was on the way, ready to deliver a manzacar that could change everything."
6. "Walk the dog" narrative and/or dialogue a, e.g. "The doorbell rang. I got up from the couch, walked across the floor of my apartment, opened the door, and saw that Bob was standing at the door. `Hello, Bob,' I said. `Hi, Mary,' said Bob." Yawn! I'd rather paint my claws and watch them dry.
7. Omniscient POV with lots of narrative. These read kind of like a fairy tale. "Once there was a man who lived in a little hut in the woods . . ." While this style can work occasionally for a specific effect, it is a hard sell.
8. Paragraphs of dialog at the beginning of a story. Once again, this can work, but not if it is basically a cheap way of getting in backstory or narrative.
9. Lengthy "quotes" from sacred texts, poems, songs, or fictional people. One or two sentences are okay, but let's get on with the story already!
10. Prolific use of adverbs. If you have more than 2 adverbs in the first sentence or 5 adverbs in the first paragraph, it makes Gremlin want to bite something fleshy.