The worst rejection letter I got was one that said "Sorry, sounds too much like Harry Potter." As a general rule real rejection letters don't say anything about the quality of the writing, because people have been known to sue editors when they say anything non-postive about the quality of the writing. Personally, I like it when I get specific comments on stories, but it's really, really rare. Wierd Tales is one place that does it. I know one of the editors, and he says that they are under constant pressure to stop being so blunt, because the executive branch of the magazine gets lots of letters of complaint.