I wrote this in response to a thread over at the Asimov's board, where I have also posted it:
In the 2 1/2 years since I started writing science fiction, I've written 50 stories. I've had 18 acceptances and 216 rejections. I have 17 stories in various slush piles currently. This is what I think I've learned about the subject:
There are seven kinds of replies you can get. In ascending order, they are:
1. Form Letter - Long Version: This is when the mag says "You're story didn't work. Here is a laundry list of reasons why you probably screwed up. Read them and think about it." You didn't come close.
2. Form Letter - Short Version. Rejection letters are not personal correspondence. In this case, shorter is better. If the letter just says, "This didn't fit our needs", take that at face value. It's not a bad story. If the letter also says "We hope to see more of your work", also take that at face value. They might be starting to notice you.
3. Form Letter - Short Version with Note. If the editor feels the "best" form rejection isn't quite enough and writes a personal note, that means you are being noticed.
4. Personal Letter - OK, this means the editor thinks you have promise. It may be short, but usually includes at least one pertinent comment. These comments may be ranked as follows:
a. Didn't work for me - when words like "unlikely" or "implausible" appear, it means you had a good dead but didn't execute well enough.
b. Liked, but not enough to buy - Means exactly what it says.
c. Liked, but not right for publication - Again, means what it says.
d. "Good story, but I just bought one like it." Drives you nuts, but it happens. To be a good writer, not only do you have to have good ideas, you need to be first out of the gate.
Words like "enjoyed", "fun" "well written" are good signs you're making progress.
Note: As far as I can tell, F&SF sends out typewritten letters to everyone, so this is meaningless for them.
5. Personal Letter - Might Consider After Rewrite: This is NOT an acceptance, but you got close enough the editor wants to encourage you. If your story was accepted, the editor will still almost always ask for some reworking, anyway.
DO NOT bother to resend and send story again to the same editor, unless it is unrecognizable after rewrite. Editors go ga-ga is they see the same story twice. Rewrite it and send it to someone else.
6. Personal Letter with Critique of Story - If an editor rejects the story but takes the time to critique it, he basically knows you're going to toss him one that will stick soon. I have this from people such as Michael Swanwick and Howard Waldrop. This is the way John Campbell worked, and Gardner Dozois, also. No busy editor is going to take the time to write a critique unless he feels he's going to buy a story eventually.
Note: Personal letters with only 1 comment are a show of good faith that the editor read your story, and this is really No. 4 above.
7. Acceptance. The editor almost always suggests changes - but it's still an acceptance.
Some magazines use rejections with checklists. If they turn around the stories fast, fine. Otherwise, send the stories to where you can get some useful feedback.
Some magazines only reject or accept - nothing in between. Their rejections don't tell you anything. Again, if they turn around stories fast, use them. Don't tie up stories for months and months for a form letter that gets you nowhere.
The only major venue I know that uses a slush pile reader is F&SF, and their rejectomancy is a sub-specialization.
Some venues - Alienskin and Andromeda Spaceways come immediately to mind - have an editorial board, so when you hear back from them, the feedback reflects some collective process. Andromeda actually divvies up the stories to a first reader, and if he forwards it to the editors, they let you know. A story there goes through two readings before it can be accepted.
Almost no one in "unrejectable". The only writers I think would t fall in that category are Arthur Clarke (who doesn't write short stores any more), Harlan Ellison and Michael Swanwick. Lots of multiple Hugo winners get rejected all the time.
The only advantage of being a pro is that it usually gets you past the slush pile reader (if there is one).