1. Formatting: Editing is a subjective business. What I may consider garbage another editor may consider gold. My opinion is basically out of your control. But formatting isn't. Take the time to actually READ the submission guidelines. Don't skim them and assume you know what you're doing. Formatting is the easiest part of the submission process. It's the equivalent of the 200 points you get on the SAT's for scribbling your name. I would say about 10% of all submissions don't follow proper format. Sigh. If you can't get the easy parts right, how in the world do expect to handle that difficult parts? So. Proper formatting. And no multiple or simultaneous submissions, please. And DON'T skip extra lines between paragraphs! I hate that. It totally interrupts my flow when I'm reading.
2. Grammar/Spelling: I can understand a mistake here or there. It happens to everyone. I won't hold that against you. My problem is when such mistakes litter your tale. Not only does it look unprofessional, but it tells me that you didn't take the time to read your work before sending it out. YOU DIDN'T TAKE THE TIME TO READ YOUR OWN WORK. What is wrong with you? Some authors will say, "I don't like doing this because if I mess with the original draft, it'll lose its vitality." Fine. Then leave every sentence and word exactly as it is . . .after you check it for spelling or grammar. If you don't care about your work enough to address another facet that is in your control, give me one good reason why I would expect your story to be any good whatsoever.
3. Cover Letters: Don't summarize your story. DON'T SUMMARIZE YOUR STORY. I'm not an agency, requesting the first three chapters and a synopsis of your novel. I'm reading shorter works. SHORT stories. Nothing longer than 10,000 words. Chances are I'm going to read (or stop reading) your work in one sitting. Why in the world would I want to know what happens? What purpose does it serve, other than to take the fun out of my reading experience. That's why I'll skip these summaries if I come across them. Also, I hate the following: "I've been reading Realms of Fantasy for three years now and I'm very impressed with its quality of stories. Given this, I feel that it's time to share my vision with the world." Buzzer sound. Shawna and I will decide if you have any vision. Don't come to us expecting publication, like you're more entitled than all the other people fighting to get out of the slush piles. That undeserved sense of entitlement really pisses me off (and FYI, I've yet to encounter a story where one of these jokers offered anything good). Last, don't list a bunch of meaningless credits in your cover letter. What's the point? I don't care about your personal life, whether it's interesting or not. Non-fiction credits don't mean that much to me either, because writing fiction is a different animal. And lastly, don't list a bunch of meaningless fiction credits. You won't impress me, and it doesn't prove you're a good writer. If you're worried that your cover letter will look empty and pathetic without such credits, you're worrying about nothing. All that matters to me is the writing. I've plucked two slush survivors accepted for publication where the authors listed none of their credits on the cover letters. Turns out they've both published with respectable venues in the past. So don't list your twenty publications with venues that mean nothing to me whatsoever. And if you have to ask me which venues are worth listing, you don't know the industry nearly as well as you should (although I'll add that not every venue needs to be of a pro level--there are some wonderful and very respectable semi-prozines out there).
4. Waking Up/Dreams: It's usually a bad idea to start a story with a character dreaming or waking up. There are exceptions, of course, but most often this is a sign that the author doesn't know how to open the tale in an interesting fashion. I'd rather see your character doing something interesting, with a clear objective in mind. And yes, there are other ways to open a story. Either way, starting it with your character waking up/dreaming is usually a terrible idea.
5. Bars: In an otherworldly fantasy, starting your story here is the equivalent of starting your tale in Cafe Cliche.
6. Office Buildings: In a modern-day setting, starting your tale here is often the equivalent of kicking things off in Cafe Cliche.
7. Elves/Dwarves/Orcs: Guys, it's been done to death. It's been done beyond death. Sure, you still see this stuff in novels, but novels are far more forgiving than short stories. Short stories put a lot more emphasis on originality. It's certainly possible that you can offer something strikingly original with these creatures, but the chances are minute.
8. A-half-elf, warrior, cleric, and mage are journeying in a dungeon: Go to the bookstore, buy yourself the necessary books and dice, and go play Dungeons & Dragons. If you want to write stories like this, there are venues better suited for such tales.
9. Humor: Guess what? Humor is also subjective. VERY subjective. Most people aren't funny. Even less people are funny when it comes to writing a funny fantasy story. How many funny fantasy authors are there? Not many. In my time at RoF, I've plucked exactly one funny fantasy from the slush (although it was published, so that's something). If you insist on sending me funny fantasies, please, let some people read your work before you send it off. Find out if anyone actually appreciates your supposed comedic genius. If they don't, what, do you imagine, are the chances that I'll feel differently?
10. Cats/Unicorns: I don't have a problem with this stuff, but Shawna does. Same difference.
Hope this helps.