Effective use of a "learning narrative" assignment in a gen ed science course
When I TA'ed a survey neuroscience course, we had a similar type of assignment where we asked students to select a topic from the course that was challenging for them personally and put together a (short) PowerPoint presentation on that topic as if they were going to teach it to next year's class. 

Overall, I thought the assignment was a good one, and the course reviews at the end of the semester generally indicated that the students felt the same way. Many of the reviews commented on the fact that the process of thinking about how to teach the topic to someone helped solidify or clarify things for themselves. Others mentioned that it helped them learn how to learn on their own by searching the web, looking things up at the library, et cetera. Establishing and building upon this skill is key no matter what you end up doing, so I consider this the biggest positive of the assignment.

The downside seemed to be that many students struggled with the "why they didn't understand it" part. It can be frustrating to not understand something, and especially when the student doesn't plan on pursuing further study in the area (which is probably the case in a class like "Relativity for Poets"), there can be a certain lack of motivation to break through conceptual or other barriers to understanding. You might need to offer some guidance in this particular area of the assignment.

Looking back, one possible improvement might have been to assign these "learning narratives" multiple times throughout the semester. I noticed that a lot of students chose the exact same topic, which might have been because it was truly the hardest topic or because it was the easiest to make a PowerPoint on. We might have gotten more diversity in responses if we split the assignment into two (or more) parts that were due at uniform points across the semester. Plus, we might have been able to reinforce the "learning to learn" aspect of it through repetition. Of course, that's just speculation on my part.