One thing that I’ve learned about through reading slush for New South is the art of flash fiction (micro fiction, short-short fiction, etc.). I didn’t have much experience with flash when I came to New South, but because we publish it regularly on our website, it wasn’t long before I’d read hundreds of flash pieces, ranging widely in their level of success.
Most slush readers are writers themselves and therefore have moments of extreme guilt about rejections. This is especially true for me when the story is close to ready or when I recognize something of my own work in it, when I realize that a problem with a submitted story is one of my own writing habits or tics.
There’s no right or wrong way to write a cover letter, and I’ve never heard of a story getting rejected (or accepted) just because of its cover letter.