Well, I'm honest. You may not like some of the things I say, but it would be with the best of intentions. I've never been outright rude to anyone I've ever beta-ed for / assisted with a story because I respect their efforts and desire to improve.
Though, I admit, I did laugh at this one girl who had me beta-read a fanfiction where she put a male character in a bright red thong because... ... yeah, no, not even in the most ridiculous fan fantasies would that character ever be caught dead in a bright red thong.