A quick post.
I slush read for a science fiction and fantasy magazine (I'm not going to say which) and I've been noticing some reoccurring patterns in the short stories I read that tend to get rejected. I wanted to share a few of those problematic themes I keep seeing. Maybe this will help you get published? I figure it can't hurt.
So here they are, things I think you should avoid in your stories so you can avoid getting rejected:
1) Your protagonist dies at the end of the story, particularly if it's by suicide.
2) Everyone dies at the end of the story, particularly if it's by mass-suicide or mass-murder.
3) Your characters have hard made-up names (i.e. Hyiliawpha, or Wjusta), particularly if there are a lot of characters. I don't think it's a coincidence that the characters in Game of Thrones tend to have modern derived names (i.e. Joffrey vs Jeffrey/Geoffrey, Margaery vs Margery, Jon vs Jon). It helps the reader keep track of so many people.
4) The Apocalypse has happened and there's no hope for the protagonist. I think we amateur writers (myself very much included) tend to confuse emotional depth with despair. To be honest, despair is really not that much fun to read. I mean, where do you go with it? I'm not saying there's never been a great story about the end of the world, but I feel like the good ones rarely start at the end (Cormac McCarthy's The Road is the only example I can think of that contradicts this "rule.")
5) The speculative elements of the story are marginal. This rule really only applies to genre submissions. Here's an example: if your story is about a family in some kind of conflict, and the family happens to have a magic broom that sweeps the house while they fight, but the broom has basically nothing to do with the characters or the story, then I'd say there's not enough speculative elements to warrant publication in a science fiction or fantasy magazine. This comes up a lot in the stories I read for the magazine.
Anyway, I hope these are of some help to you. I'm not going to say these are hard and fast rules; a lot of it depends on execution. But I've yet to read a story I liked where the character committed suicide (you'd be surprised how often that comes up in these submissions).