What's interesting, is that I rejected an awful lot of good stories in the past few days, stories that in some cases had very little to distinguish them from the stories I still have. And one of the things that you wouldn't expect to make a big deal is prose style -- but it was the deciding factor in quite a few of those stories. Because if a writer gives you a story with a weak ending, or an overlong beginning, or even a story that isn't sexy enough -- those things can generally be easily fixed. It's worth asking for a revision, seeing what they can do. But if they have a clunky prose style, a weak grasp of language -- well, then you're in trouble. Because it's going to take way too much time for you to line edit every single line of the story into smoother prose. And odds are, if you send it back to them to revise, they won't be able to do it for you; or, if they can, they can't do it quickly. So those good stories with that problem -- I tend to send them back, and hope that with more practice, those writers will learn to smooth out their styles, get a better grasp of language.
Maybe other editors would choose differently, but I don't really know what else to do with that situation.