When we have gotten to know a student somewhat through her writing, or when we are reading a sufficiently long manuscript, we can sometimes notice particular bits of what I want to call resonance: places where I'd say that the writer has gotten a bit more of his or her self behind or underneath the words. Often these are little changes of tone or eruptions or asides or digressions--even lapses of a sort, but I experience them as pieces of added weight, richness, resonance, or presence (even if they are bits of irony, play, metaphor, or even silliness). Some important dimension of perception or thinking or feeling formerly kept out of the writing is now allowed in. Good musicians often get a bit more of themselves into or underneath the notes.
I'm not calling these places "good writing." They may even be places where the writing breaks down. That is--except for exceptionally skilled writers--resonant passages are often holes or cracks or disjunctures in structure or voice the writer was trying to use. Often the piece is going to have to get worse before it can realize the potential resonance that is trying to get in. But I think I've noticed myself and other writers benefiting from having these passages pointed out. Often the writer can say, "Yeah, I can feel something different was going on as I was writing those passages." I believe that it helps as writers if someone is good at noticing those passages and then says, "Try for more of that." This doesn't necessarily help in revising this piece (especially if the deadline is near), but as a path towards gradual progress as a writer.