A 60 on the paper means it was competent but I didn't find much there beyond what was explicit in the sources, or else there was a lot of confusion in the writing. A 70 is good work, with some synthesis and analysis of literature content, and an 80 meant especially clear or complete exposition of the area. I was prepared to go to 90 but no one did brilliant work this time around.
The paper actually made very little difference, except in plagiarism cases, to your relative standing in the course and therefore to your grade. It was intended simply to benefit the writer.
Revised histogram after adding late papers: