I learned more from reading slush at the California Quarterly when I was in grad school than in any course I was taking at the time. We received 40-50 manuscripts a week, and we didn't accept a story in the first four months I was there. It wasn't that it was all bad--although heavens knows that over half the stuff was immediately rejectable--but that so little of it stood out. Here's some usable info I learned from the editor's side of reading slush. If your manuscript exhibits any of the following, it's better than half of the slush pile: 

          1) If you use action verbs in the first paragraph. 
          2) If you don't use a cliche in the first paragraph. 
          3) If you don't use unneeded words in the first paragraph. 
          4) If you name things specifically in the first paragraph. 

A lot of manuscripts managed to use no action verbs, to use at least one cliche, to throw in unneeded words, and to not name a single thing specifically, all in the first paragraph, and those were the hardest to keep reading until the end. 

What I learned over and over and over again, through weeks of reading slush, is that professional, readable writing is recognizable in the first paragraph. Getting to the second page without running into a single groaner was such a relief that I'd sometimes read the first page of such a story to anyone who was near just so they could hear competent prose. 

Writing well at the sentence and paragraph level is what I keep pounding into my students and workshop members. That's why I think studying poetry can be so helpful: poetry is all about sentence level decisions. At any rate, that's what I learned. My guess is that if you have a chance to read slush or to read for a contest you might learn something different, but, no matter what, do it. It's a great, educational, professional move.

Here's something else I learned too. Over half the authors in the slush pile made multiple mistakes in their first page of the sort I described above, and that made them easy to reject, but that means a good bunch of them didn't. So how do you choose from among the competent? 

What really kept me reading as an editor, and what was required in a manuscript we eventually bought, was that something interesting occur on the first page. That interesting thing could be an event, an image, a phrasing, an odd connection, a bizarre situation, a simile, etc. It had to be something that widened the pupils, or made me catch my breath, or made me say, "Oh, cool!" What's amazing about this interesting quality, by the way, is that it doesn't require genius (I don't think), but a willingness to be focused and original. Focused on the specific reality of the story and original in language and vision. The writer who could be interesting on the first page was much more likely to continue being interesting than the ones who weren't.

I even think I know why so few stories had this quality: they came from writers who didn't revise enough ("revise" as in "to see again"--not proofreading). If you think about it, most writers don't have an absolutely clear idea of what their story is when they write their first paragraph. They learn about the story as they write. Their clearest vision of what the story is comes to them when they're on the last couple of pages (although not always on the last paragraph--endings are hard; they really require the writer to know what the story was about). So they send out these stories with their weakest writing still at the beginning where their characters, setting, situation, conflict, language, tone, mood, etc. were at their vaguest.

Sometimes what I think is the problem for the writer is not an unwillingness to be specific, but a lack of knowledge about the specifics.  When I workshop stories I'll ask questions about specifics, like "What are the shops on the street?" or "What is the pattern on the couch your character is sitting on?" and more often than not, the writer won't know.  The writer hasn't thought about it.  Of course, you could say those were unfair questions, the writer can't know everything, right?  I agree, but these writers didn't know anything beyond what was on the page.  They hadn't fully occupied their fictional world, and it showed in their level of detail.

I think a writer can get a lot more milage out of the tiny thing imagined well than the huge thing imagined poorly.  What convinces the reader that the story is real is the tiny detail.  It's Prufrock's description of the girl he's going to meet, 

          Arms that are braceleted and white and bare

          [But in the lamplight, downed with light brown hair!]

It's Robert Hayden's evocation of the father in "Those Winter Sundays":

          Sundays too my father got up early
          and put his clothes on in the blueblack cold,
          then with cracked hands that ached
          from labor in the weekday weather made
          banked fires blaze.

Both descriptions get miles of good work by the tiny details, the “in the lamplight, downed with light brown hair!” and the “cracked hands that ached from labor.”

At any rate, this is a long post that says, among other things, be specific.