All of the editors at e-spec are also writers and we've suffered our share of rejections. It would be really great if the rejection notices we got from editors identified why our stories were rejected, but it rarely happens. However, being on the other side of the table, its easier to see why, as authors, we don't get the personal attention we'd like. Even being a relatively small magazine, we get a lot of submissions. Unlike some other magazines that are out there, we're pretty good about reading your whole story--not just your bio and the first few paragraphs. So, it takes time just to read and reply to submission. If we included a critique with each one, we'd fall hopelessly behind. We strive for a quick turn-around time.
So, while, in most instances, we can't tell you why we rejected your particular story, we can share common reasons for rejection. Here are a few of the major reasons, and maybe some of my co-editors will want to add to the list:
1) We've seen it before. We get quite a few submissions from people who do not seem to be familiar with the genre they are writing in. So, the plots of many of the stories we get don't hold our interest because we've seen the same or similar terminate before. Sometimes, we get stories that are hard to put in the rejection pile because they are really well-written, but they're just not unique enough to make the cut.
2) We saw it coming. Most of us are not opposed to an "O'Henry" twist at the end of a story. That being said, it is very hard to pull one of these off. Many of the stories with twist endings have twists that have been done before (see #1 above) or the whole story seems like a set-up for the twist ending. Even if the ending is really good, the rest of the story better be engaging, too, or it probably will not make the cut.
3) We didn't see it coming, but we should have. We get a number of stories are good up until that last line. These are stories where the ending is just not satisfying. This is not to say that every ending needs to be a twist (see #2), tied up in a neat little bow, or happy. However, we don't want to feel like we've been left hanging, the plot has not been resolved to a reasonable degree, or we don't know why we just read the story.
4) Poor writing. This one is a bit tricky to talk about because it is hard to define. We're not talking about grammar and spelling (which is important, but not usually the problem). Instead, we're talking about the tools writers use to make a story engaging. Even with a great plot idea, you need to be able to set it out in a way that pulls the reader in. Common traits that could go in the "poor writing" category include: Too much detail not important to the story, too little detail about the characters or setting, too much narrative (i.e. telling rather than showing), writing that is vague or confusing, too many adverbs, too many exclamation points, unrealistic dialogue, not enough dialogue, poor character descriptions, too much backstory, and flat or trope characters.
Given all this, we'll probably need to post something about what we do like in a story. That will be upcoming.