I'm amazed at how many authors send in stories without a cover letter. Nothing, just a blank email with an attachment. It blows me away. How rude. I'm taking precious moments out of my busy day to read your story and you can't bother to at least greet me and then say thank you? Then there are the bare-bones cover letters that I can tell are generic, as in, Dear Editor, please consider this story. No story title. No word count. Pretty much garners the same feeling from me. If you don't have time to write up a cover letter for your story, why should I take time to read it? But I do. And then the other extreme: the letters with TMI, Too Much Information. Every single publication they've ever made, an autobiography, and then the dreaded story behind the story. I received one the other day that had no information about the story they were sending (besides, please consider this story), and then 4 pages of biography, background, publication listings, and I don't know what else. I didn't read it. Nor did I do more than glance at the story. Sorry, but I had visions of someone very difficult to work with.
My biggest pet peeve is no word count in the cover letter. Why? Because I'm lazy. Sometimes I only have a short time to read one more story, and I know I can read a 2k story pretty quickly, but a 4-5k story is going to take longer. I don't want to have to open the document to find out how many words the story is. Please, be kind. Let your slush reader know how many words your story is.