There are actually pitched battles going on in fannish communities over that very thing -- what needs warning, what level of summary to give, artistic effect vs. triggers. It varies by fannish subgroup and forum -- obviously one goes with whatever one is personally comfortable with on one's own website and own journal, but there may be different standards in any given fic community.

The AO3 is designed to be about as flexible as possible in terms of warning options, since it's intended as a fanworks archive, so the author can share what they feel needs sharing, and the reader can decide what level of warning they want.