You've read the guidelines. You've taken a look at the magazine. (They always say "read the magazine for a good idea of the kind of work we accept.") But you are confused. You think your work is just as good as what you are reading. It could fit, right? Why not?
I'm now on both sides. I keep sending work out. But I'm also reading submissions and choosing work to include in *82 Review. In the past three months I have been startled by what I have read. The following are some points to be aware of when you submit your stories and poems. I've tailored them to my own taste, but that's not really the point. Just be hyper-aware of what you are doing, not necessarily as you are writing, but after you write.
The typical amount of submissions that are rejected per cycle: 70-80% (it is much higher with well-known magazines). Submissions I accepted for the first issue: 20%. I was shocked when I saw how much I had to reject. Now I know why it is hard to get work accepted for publication; it has to be original and interesting and fresher than anything else submitted. It has to have a spark, a knowing look, a little wink or punch in the stomach that leaves you satisfied ("yeah! I needed that!"). Something has to happen.
Here is a list of common tropes to avoid and some alternate suggestions. These are not absolute rules, but perhaps you can increase your chances of getting published by being aware of them.
Please no: stories with run over kittens (I've seen more than one!); pieces that contain abuse or cruelty with no redeeming reason, message, or knowledge gained
Better: stories with an edge (reader meant to feel slightly uneasy, but trusts the writer will lead them to important ground); something changes; something new is understood; reader feels it, gets it, is changed by it
Please no: poems about love (unless you are Shakespeare. And if you think you are Shakespeare you probably won't be published, either)
Better: poems about an object or place or time where love is shown subtly in the background or the use of language (verbs, in particular) conveys the emotion
Please no: pieces that contain accounts of visions via mind-altering substances
Better: geeked-out observations that are accessible and exciting
Please no: accounts of seeing the perfect unobtainable woman/man
Better: pieces that show a longing through the use of language and layered metaphor
Please no: writing about writing, or writing about thinking, or writing about thinking about writing, or writing about thinking about not writing
Better: a story that might have some meta elements or refer to a book or library, but is about something in the physical world
Please no: jokes
Better: a skewed look at the world that makes us see the world differently and laugh at something that was there all along, but that we never really noticed
Please no: explanations
Better: matter-of-fact story or poem that shows us the characters through their speech, how they dress, where they go, what they eat, etc, but doesn't explicitly tell us who they are
Please no: poem about the moon; poem that uses all the colors of the rainbow; poem that uses all the letters of the alphabet; other gimmicks that are more about "look at me" rather than "I'm trying to show you something."
Better: poem about a collection, poem about an ordinary object made fascinating that has some emotional depth or layers
Please no: first world problem or problem someone who is privileged would have, such as, "My Mercedes broke down"
Better: "The Mercedes that I stole broke down."
Please no: realistic narrative that goes into excruciating or mundane details about divorce, dating, college life
Better: surreal, dreamlike, experimental narrative that may include actual events somewhat disguised, show a change, make the reader care about the character and really want to know what happens next
Please no: gratuitous raw or shocking language
Better: raw or shocking language used sparingly to illustrate a point or service the story
Please no: outsider or outcast wandering in search of self
Better: the outsider or outcast getting caught up in someone else's life or problem; it is much more interesting to see how the character interacts with someone or something else, how the character deals with the new problem
Please no: painful wait for someone's death (unless you are Faulkner) or the ruminating or anguish afterwards (yes, I know from personal experience: it hurts!)
Better: convey loss by having the character handle the possessions of the loved one and describe them or relate a story about the loved one's life; tell indirectly of the death and emotions through another event; have the feelings unfold as the story does
This is my own (humble) opinion and based on what I like to read. After you write, I only ask that you find a way to include the reader.