When you're writing a story that you hope will one day be published it is important that you are aware of what editors are looking for. Why waste your time and efforts submitting to a publishing company that won't even glance at your manuscript because it's not the type of story they're hoping to find?
Equally, it is important that you keep in mind stories that they just don't want to see. This is not necessarily because the concept is a bad one. It could simply be because they have seen a similar concept time and time again.
This list is part of the submission guidelines for Strange Horizons, an online speculative fiction magazine. I just read through them and checked them off one by one (after reading an interesting post about rejections by author Eden Royce here). I am now confident that my story is not something that they won't want to see.
Check out the submissions guidelines from Strange Horizons here, and don't forget to research each and every company before you send in a submission.
Happy writing, folks!