1. Do not, under any circumstances, submit a story in colored font. Black only. Please. My eyes can only take so much.
2. I'm finding I really detest that old editor's standard, Courier New (or Courier, or Courier whatever). I much prefer Times New Roman.
3. And while we're on the subject of fonts, do not submit a story in anything but the fonts recommended by A) EVERY standard manuscript format guideline or B) the submission guidelines of the market to which you are submitting. B, however, is most important. I mean, I suppose there may be some publisher who really loves to read stories in Harlow Solid Italic, and if there is that publisher would certainly make that known in his/her guidelines. I mean, there may be. Right? Maybe?
Or not. But if there is then you should do what she/he asks.
4. If you are NOT a native English-speaker who is writing in English, I cannot overstate the need for you to have a native English-speaker read through your manuscript to make sure it reads well in English.
5. Big blocks of text, with no paragraph breaks, are a turn-off. Honestly, I have yet to see a story that uses such a style that wouldn't be improved by adding a few hard returns.
6. Short story cover letters and novel cover letters are not the same. If you don't know the difference, Google it. In a nutshell, however, short story cover letters should NOT include a synopsis of the story. It's a short story. If I can't figure out what it's about by reading the first page or so (even the first paragraph or so) then you haven't done a very good job of writing it. All I want from your cover letter is a polite presentation of your story, a short list of some of your most recent or most notable publication credits, and a 'Thank you for your time.'
7. You do not need to add personal copyright information in your story file or cover letter. Copyright, by law, is implied. Also, a reputable publisher who has been in operation (and publishing regularly) for more than, say 6 months, is not likely to steal your story. Our reputations are important to us.
8. I'll say it again (and again and again and again), you absolutely MUST read and heed submission guidelines. You must.
9. I once frequented a writers' forum in which someone stated that if your grammar and punctuation are good you're ahead of 90% of your competitors. Not actually true. In my experience--and I've been handling slush for nearly 10 years--most writers can competently handle the mechanics of the English language. What most lack, quite honestly, is finesse.
10. The F-word is being tossed around like confetti these days. I, personally, am not impressed. It smacks of a crudeness and lack of refinement that I find unappealing at best.