This post echoes some of the same thoughts I had when I started reading slush fiction
I'll also add:
* Make sure something interesting happens on the first page, and preferably in the first paragraph. A slow build can be good, but is often a harder sell; there are fewer stories where the tradeoff is worthwhile.
* Do your homework. If you're writing about history, or science, or some other topic I know or have casually brushed against, and I spot a factual mistake in your story, that will cost you the sale far more often than not.
* Include at least one character in every story, preferably the protagonist, who is really worth spending time with and rooting for. If everyone is unpleasant or annoying, who wants to spend half an hour or more with them? There are other, more personable characters elsewhere in the slushpile also clamoring for the editor's attention. *chuckle* I actually learned this one before I ever got to a slushpile, just deciding who I wanted to write about and have in my head.