Not all that long ago, I became a volunteer slush-pile reader for an online magazine. It has been an educational experience for me. And while I am not a bona fide professional when it comes to telling anyone why they did or didn’t get accepted to any particular publication, I do have some general advice. Maybe it’s not so much advice as a plea. (English isn’t easy, folks, but it’s not quite as hard as some people are making it seem.)
First. Master the art of short words. Few things look worse than the misspelling and misuse of short words. Typos do happen, but you’ll only be forgiven for a precious few of those. And it can be painfully obvious when a writer doesn’t understand the difference between its and it’s. Or worse, the difference between then and than. Treat these words as you would public and pubic. Blush if you swap one for the other, watch them with hawk-like scrutiny.
Second. Use profanity sparingly or, better yet, not at all. Once you’re an established, known author, drop all the F-bombs you want. Until then, you run the risk of looking lazy, crude, or unimaginative. Every single time you want to use profanity, stop and ask yourself “what do I want more, to shock someone, or get accepted?” If you want the latter more, choose another word. Sure, the slush-pile reader who sees your piece might be as foul-mouthed as any sailor. Or it might be someone as prim and proper as your least favorite auntie. Bottom line – play it safe. No one’s going to reject a piece over too little profanity.
Third. Unless you have an MFA in Creative Writing, get someone who’s reasonably well-read to proofread your story. (This is a good idea even if you do have that MFA.) It’s not as glaring as the then/than problem, but really pays to not make the mistake of using “sodder” for “solder.” If you must use the word “cajones” then don’t spell it “kahonees.” Remember that “per se” is a fancy way of saying “in itself” while “perse” is an obscure way to say “dark bluish gray.” There’s no end to the weird words that trip us up, so it really pays to have a second set of (knowledgeable) eyes take a gander at what you’ve written.
Having said all that, I’m cringing as I post this. We’re only human, and I’m probably putting several mistakes on display even now.
Kelly Horn 
I don’t envy your being a slush-pile reader. While you do learn a lot, I’m sure, it’s also painful to read some really poorly written stories (just be thankful you’re not reading poetry…bad poetry is far more painful than bad prose and it’s also much easier to write bad poetry than bad prose). :}
I wouldn’t read poetry submissions of any kind, ever, not even for a paying gig. It would be unbearable, at any price 😉