Anonymous and Shopfront talk here about triggering, automatic thinking, and the effects.
shopfront goes into more detail here.
Warning: Both these contain links to and descriptions of mental illness, self-harm, and suicide.
Like impertinence's essay Sexual Assault, Triggering, and Warnings: An Essay (Warning: Very explicit discussion of sexual assault and the nature, anatomy, cause & effect of triggers. Is itself triggery.), both are frank descriptions of how triggers work for them and the effects of this kind of mental illness. The first link I've been following since the thread started and highly, highly recommend reading the conversation; the second link I read this morning and am still thinking about.
All of these reflect, with specific examples and reasons, why a warning system for readers with triggers is so important and the absolute necessity of writers making sure people have the tools to choose whether to read their fic.
I recommend reading both of these.
Months ago, someone did a review on a fic of mine that surprised me regarding how I structure a story (I am not one to give away information early in the story if I can piecemeal it out in drabs before dropping the major arc right on top of the reader). It wasn't something I'd noticed consciously, but suffice to say, the fact I tend to use a single line or two from a story as a summary should tell you how much in general I loathe giving away anything I don't have to on a fic before it is read. I really hate it. It drives me nuts. This is where I admit, yes, apparently I do have a goddamn artistic temperament about this sort of thing, and you have no idea what it cost me to say that, because I never thought I did.
I do not warn, I barely summarize, and sometimes I leave out pairing codes if they aren't important to the major plotline (yes, in fact I have gotten some comments about that before). If I could get away with it, I'd leave out pairing codes, spoilers, and possibly the title, but ASC habits are impossible to break. I can barely leave off Archiving in my headers, it's that ground in.
I would not have said three days ago that it would be easy to stop, go back, stare at my fic, and decide if a warning is required. I was wrong; it's shockingly easy to do so.
The reason we have a spoiler line in headers is twofold: one, so the person reading doesn't get spoiled for a future episode of a show/spoiled for the movie/spoiled for the book; two, at least for me, so they have context if I'm depending on either some fairly obscure canon or something specifically in canon is being used extensively. Very few people argue spoilers destroy stories, even though spoilers are more likely to make me skip a story than a warning. It's also something we rarely argue. As a group, we slam down on that sort of attitude kinda viciously, which I am all for.
Having a spoiler line does not destroy your story. Somehow, strangely enough, giving the reader that much context does in fact improve readership. They can find what they are looking for. And if they aren't ready to read it, they can skip it for later/never. They get excited if it is about Trinity or Eye of the Storm, if you are in SGA and like that sort of thing.
The reason we have summaries is to tell the reader why they should read the story. I'll be honest--I am not good at this. I am bad at this. I grab a line that looks interesting and throw it up there. I cannot summarize. But even doing that tells the reader something of the story before they read it. I am more likely to skip a story based on the summary than the warnings.
Having a summary, unless you do a plot analysis in three sentences, does not destroy your story. They can improve readership. They give the reader a taste. They get excited if you mention the Slash Dragon or Arthur's Quest for a Goddamn Special Flower While Merlin Moans (aka seriously, this is canon? Thank you, BBC) if you are in Merlin and like that sort of thing.
I could repeat this with title, pairing code, fandom; if you continue to think warnings are some kind of be-all and end-all to make or break your artistic vision, then it's not a structurally sound artistic vision. Warnings, like title, author, pairing codes, spoilers, summary, aren't a law. You leave off a spoiler warnings, someone finds out that Chewbacca is actually Luke's father. That's worst case, and fandom goes to war for it. Worst case scenario in warnings...
Wow, I just went to check something. I can tell you what that is. For me, it was done on December 3, 2007. I posted a fic with implied violent sexual assault, fairly graphic torture, mind control, and some really creepy temporary mutilation along with drug use and this, this was my summary:
Shifting his hands weakly, John stares up at the forcefield lines that cross below the ceiling and wonders what the hell happened.
Worst case scenario, someone read that who wasn't ready for it and suffered because I am just that artistic.
Warnings are easy.