Aspects you might consider include how to structure an argument or how to support a position with appropriate evidence. For example, you may quickly realize that your student has considerable trouble constructing a coherent argument. This takes considerable time and knowledge of the conventions of writing, and it focuses attention away from the student's ideas. In fact, many writing instructors and researchers argue that this form of commenting is most beneficial for students who gain the valuable knowledge of how their writing is seen through a reader's eyes. This method is especially useful for commenting on drafts.
Students can also share drafts or papers and give each other responses.
Many teachers feel students benefit more if comments are focused on ideas, but grammar, syntax, and the like are still important. Resist any impulse you have to correct their work for them and instead encourage students to take responsibility for these aspects of their own writing. There is certainly no shortage of appropriate and helpful texts which students are perfectly capable of using. You can also make grammatical and syntactical comments on one paragraph to give the student an idea of the kinds of changes that are required. By limiting this type of comment to one paragraph, you save time and avoid overwhelming the student.