I like what Jonas said about the goal of the lesson. I don’t teach creative writing, but I often assign essays in my social studies classes. I typically use a simple rubric based on the student’s understanding of the material as demonstrated by the essay. Here’s one such assignment I gave recently:
“Write an essay showing how the dynastic cycle, the role of nomads, and the Chinese sense of superiority have been demonstrated is several periods of Chinese history.”
This assignment had three major topics—the dynastic cycle, the role of nomads, and the Chinese sense of superiority. I rate the student’s treatment of each topic on a scale of 1–5 or 1–10. The sum of the three ratings is divided by 15 or 30 (the maximum possible rating) to determine the student’s grade. I’ll sometimes add a rating for mechanics and/or proper essay form.