Here's two questions: what sort of content do you personally find disturbing in your reading; and when writing, what indications if any do you prefer to leave the reader?
Heck, I've ranted about this a lot in the past. I remember having a very visceral reaction to a story where Jaheira was disemboweled in a very graphic way, along with several other NPC's, without any warning given. I was quite young then and by now have more tolerance towards graphic violence in stories, but there are still times where it makes me uncomfortable. I think my issue is that I always keep wondering about the writer's intent whenever I encounter graphic violence or sex. Does it contribute to the story or is it just titillation? 9 times out of 10, I find it doesn't add anything at all to the story and is just cheap means to shock or gross out the reader. Don't try to point that out to the writer, though - as far as they are concerned, they are writing deep art that isn't for faint-hearted weak-stomached pussies with no appreciation for their greatness. Ok. Fair enough. I move on to another story, no big deal.
As far as my own writing goes, while I do include some violence and some smut, I don't think that I have ever managed to gross out anyone in 10 years, so it's all probably very tame. I have managed to upset and depress people by inflicting great emotional anguish and psychological torment onto my characters, for which I am rather proud.
I find that takes a greater degree of writing skill than graphically chopping off someone's private parts and making them eat it. (But hey, I'm not a published writer, so maybe I'm wrong)
Cutting enemies into bloody chunks is a perfectly healthy part of fantasy worlds, but the least description of kissing or sexuality is evil and will corrupt the children.
Yeah, I know there's a surprising amount of people who honestly do believe this. If that's not a double-standard at its finest, I don't know what is. Ah, thank you HBO for Game of Thrones. I can have my violence and I can have my mindless fucking.
Rape and torture and wading through otyugh slime are the sort of things that always make a story super-realistic. Only shallow and immature children can't appreciate grim dark unrelenting dark darkness.
Yeah, and I've seen people who state this fail pathetically at writing rape or torture victims. They simply have no clue about realistic reaction to such horrific experiences.
I warn for everything that could possibly traumatise someone, somewhere, anywhere--hitting people, gay people existing, quoting Swinburne, kissing unshaven Anomen, characters people don't like, dirty clothing, haircuts, not speaking a foreign language, surprise endings, unexpected goldfish insertion. If your warning list's not longer than the fic itself, you're probably going to hurt someone's feelings.
With the growing self-entitlement of the internet population and the amount of people with the sole intent of searching for something to get offended about, I can sort of understand the temptation, but ultimately, this sort of apologetic pandering is the worst kind of way to deal with these people. And even if you would write two hundred warnings, you'd probably forget to warn that your character can be seen wearing socks in one brief episode, and thus will still end up upsetting the Sock-hating minority league who sincerely believe that wearing socks is the greatest sin in the eyes of their sock-hating deity.
I guess my own rule when deciding about inclusion of warnings is, basically, "Don't be an idiot". Not wanting to be presumptious, I can however see that this rule probably won't help 75% of the human population.