As you pull together your goals for the course and the papers you visualize students writing, you need to think specifically about what you want to spend your commenting time on. If you know students will struggle getting enough details into the format you've specified, then put "details" high on your list of evaluative criteria. If you expect that organization will present problems, then emphasize organization. Or focus. Or audience appeal. Or whatever skills you want to spend your time giving students feedback on. You'll know, based on your curriculum within the course and within the sequence of courses, what students need to master at each point in their learning process. Focus on those elements in your assignment and let students know exactly what you'll be spending time on. Students will work especially hard on those features of their papers, and the final products will demonstrate just what students can do with focus, organization or detail. Both you and the student will win because the student will get to work on and learn from working on what counts most in this assignment. And you'll win because you won't be distracted as you channel your energies into evaluating the specific features you set up.