But if there's ever any doubt as to whether a comment crosses the line, I think a reader should just avoid making it on general principle.

I agree. I also agree that communities might want to consider expounding on what is acceptable or unacceptable versus expecting everyone to know unwritten rules. It would not only help the readers, but help the writers, as well. For instance, although my hd_worldcup story was well received, I probably would have thought twice (or thrice) about joining the competition if I'd realized that criticisms might be part of the experience.