Before learning can be assessed, best practices in curriculum design dictate that the instructor provide clear expectations for the assignment. The easiest way to do this is through a rubric. In the screen shots below, I have made use of of the rubric capabilities within the Canvas LMS. (Click images to view larger.) Rubrics are provided in order to assess discussions and other assignments. From a curriculum design standpoint, they have been designed to ensure they measure student attainment of knowledge and skills that lead to them attaining the course outcomes.
Discussion posts are required to be substantive. Learners are provided instructions on how to access the rubric, as well as a link to a downloadable document that describes the attributes of a substantive post.
When asked to complete written assignments, learners are once again provided a rubric, as well as a link to a downloadable document that describes the attributes of a well written response. In both discussions and other assignments, learners can choose to click the "view longer description" link to see a pop-up description of the criteria. (A function of the Canvas LMS.)
One further note, the written response offers learners a choice in how to complete the assignment, thus differentiating the assignment to meet the needs of all learners. The learner can choose to submit an essay, a video, or use an online tool to complete the assignment. This effectively differentiates the assignment for multiple learning types.