Here's a pet peeve of mine about how people post fanfic.
It really annoys me when people rate all of their stories R (Mature) for "safety", regardless of the actual content of the fic. Apparently some folks do this under some (misguided?) attempt to make themselves safe from I guess criticism just in case they accidentally rate one of their fics too low, and maybe some parent or something thinks a story rated PG-13 (Teen) should have been rated R and complains, leading perhaps to their account being suspended, or being sued, or something.
IMO as a reader, rating every story the same makes the rating useless, as it does not tell the potential reader anything about the content at all, so the rating might just as well not be there, for all the good it does.
ETA: to clarify, my pet peeve is when a person uses the same rating on all their fics, so the "rating" becomes meaningless because they rate a fic the same regardless of whether it contains G-type content or R-type content.