I like to tell my students that, for the writer, the classroom can be a dangerous place. It can make you think that you are owed a reader. When my students give me work, I read every word. I read stacks and stacks of papers. I write questions in the margins. I tell them how they can make their work stronger. Sometimes I even give them a grade (note that this is the last thing on my list but likely the only thing on theirs).
This is the only way I know how to be a writing teacher, but it doesn't mimic the writer-reader relationship very well. A colleague once told me about marking student papers with a line and writing, "this is where I stopped reading." I think he might have been joking, but sometimes I wonder if that wouldn't be the best feedback of all. This is where you lost me. This is where I stopped caring. This is the moment I thought, Oh Jesus, shut up.
I don't see me ever taking such a hard line with students, and I'm certainly not doing it as an editor. There's a reason we rarely send specific feedback, and it's not because we're so busy. I mean we are so busy, but also, writers get pissed. I'm not opening myself up to the backlash, but think how amazing that would be, if at the bottom of the decline letter, there was just a short note that read: You lost me on the second page, line 3.