When I was a student I had this same experience peer-editing prose. In reading a classmate's paper, I found endless nits to pick about syntax and minute points of diction or style, but much less to say about the actual argument. The main reason for this is just that it was easier. It doesn't take much effort to call out bits that strike one as unaesthetic, but evaluating the substance of a paper requires that I really think about what it's saying.