No matter how much you want to improve student writing, remember that students can only take in so much information about a paper at one time. Particularly because writing is such an egocentric activity, writers tend to feel overloaded quickly by excessively detailed feedback about their writing.
Moreover, because most writing can be considered work in progress (because students will continue to think about the content and presentation of their papers even if they don't actively revise), commenting exhaustively on every feature of a draft is counter-productive. Too many comments can make student writers feel as if the teacher is taking control of the paper and cutting off productive avenues for revision.
Focusing your energy when commenting achieves two main goals:
It leaves students in control of their writing so that they can consider revising--or at least learning from the experience of having written the paper.
It gives teachers a sense of tackling the most important elements of a paper rather than getting bogged down in detail that might just get ignored by the student.
Typically, we recommend that teachers comment discursively on the one or two most important features of a paper, determined either by your criteria for the assignment or by the seriousness of the effect on a reader of a given paper.