This is exactly what I liked seeing from professors on my papers:
So the graduate paper deserves extensive comments, like those of a peer-reviewed article. I just write about a single-spaced page outlining the strengths and weaknesses of the paper. Everything is fair game, from the format and prose-style to the substance of the argument. I’m looking for a strong, distinctive thesis backed up by some kind of convincing evidence. What I really want to see is a level of engagement that leads to some strong thinking: ideas that I would not have come up with myself. I don’t really hold back in outlining the weaknesses. It is not really being “nice” to do this. I might exaggerate a bit on the strengths just to provide some encouragement, but I am the professor who is going to kick your ass on the final paper. Yes, I am that guy.