I'm not a slush reader, but at the Oregon Short Story Workshop, we all took the part of editors, reading for an unnamed anthology. The slush pile consisted of stories we wrote during the workshop, our submission stories, plus stories we submitted under pseudonyms, and a few that Dean Wesley Smith threw into the pile. (Some with his name, some under pen names.)
We all worked independently. We could each choose our own theme.
And we read.
We were looking for quality, of course, but we were also looking for stories that worked together. We were also looking for stories that would keep us within the given budget for dollars and word count.
And we came out of this exercise enlightened.
It wasn't so much that we now had full sympathy for all editors everywhere. (Dream on.) It's that reading so many stories in such a compressed timespan showed us how alike so many stories are. These were all decent stories. Competent. But there were those that stood out. Why?
Openings that deftly captured the reader's interest. Characters that intrigued. Situations that made her curious and dragged her along a path of words into the heart of the story.
It's not so much what the stories were about, but how the writer told them.
When writers start out, they spend a lot of time learning about the rules about grammar and paragraphing. They learn about POV and punctuation. They become aware of the tools and the basic ways to use them. This is good and necessary, but it's just a start. Stories at this level won't cause anyone's eyes to bleed, but they also won't catch and keep a reader's attention.
Over and over, Kris and Gardner told us that there's a grayness to the slush pile. Writers who sell, who break through to the next level, their manuscripts don't look like anyone else's. Sure they follow standard manuscript format, but the paragraphing is different. The prose twist and twirls. Even the punctuation can send you off-tilt. And yeah, sometimes it breaks the rules, but unlike the newbie's manuscripts, these "mistakes" are for a reason.
Stories that take risks.