Some thoughts on the various arguments against Choose Not to Warn, that road_rhythm and I refute:
#1. I come to fanfic for comfort and familiarity; I don’t want to be surprised.
These are not the stories you're looking for. Really. I don't want people to hurt themselves on my writing, and that has happened. People who are touchy about Choose Not To Warn probably shouldn't be reading me. People who don't want to be surprised definitely shouldn't be. Try the Fluff tag, it's pretty reliable comfort reading.
That line in my profile is not a joke: "Warning: The content of this blog may cause permanent damage to narrow minds."
#2. Warnings aren’t spoilers; if your plot relies on “twists” and revealing them detracts from your story, it was a pretty poor story to begin with.
Thousands of years of great literature disagree. But I'm guessing the people who make this argument aren't English majors.
#4. There’s no such thing as a ‘blanket warning’.
Sure there is. I have several.
Because X Is Always a Warning -- where X can be a variety of things, like "the inside of Shiv's head" -- names a category of content generally known to be disturbing, but not always for the same reason. Sometimes it is Shiv's bad tape saying very triggery things, other times because he likes to hurt people, occasionally because he enjoys playing in blood. The kid is fucked up. So he needs a warning about that in general, because he might be screwing off in ways I didn't think to list specifically but the Because warning tells everyone this is heavy stuff.
I do some other things that I hope people find helpful. This is my general scale of warning that gives you a rough estimate of intensity:
(no warning) -- It's unlikely to upset anyone.
Warning -- Just a heads up that you might want to know some stuff.
Warning -- Some people may not want to read this because of its content, or will read it only if it's a major plot point.
WARNING -- Bigger issues mean you may want to think twice about whether to read this.
WARNING -- Please make sure that you are in headspace to handle difficult and uncomfortable ideas, and have time to handle the aftermath if the story knocks you on your ass.
Other words and phrases: intense, touchy, controversial, may disturb some readers, likely to disturb many readers.
And then there is WARN ALL THE THINGS. I put that on when there are multiple things that usually constitute major warnings and the sum total is likely to bother a lot of people. I don't want readers to stumble into, say, mad science torture of children without knowing what they are getting into. I have put up the crossbars and blinky lights and red flags. Enter at your own risk if you appreciate literature that is intense, controversial, and sometimes gives people nightmares. Only once has a reader blown past that warning and then flounced out because they didn't like what was inside. Guys, come on, I can't bubblewrap the internet for you. If that's what you want, buy a kiddie filter, it'll block most of the ooky stuff for you.
I won't tell you not to mountainbike down a steep slope. I just want you to have some idea of when you only need kneepads and when you need a helmet with a bite guard.
"Canon-typical" is an interesting term because it varies. It literally means that something is (or is not, for the "canon-atypical" version) within the standard range for that series. Look at the range of stuff I write. Polychrome Heroics has mostly upbeat material, a lot of crunchy stuff, and a few truly dark pieces; but you should always get the sense that it's a better world than here and good usually prevails in the end. Frankenstein's Family is gothic fluff. It uses a lot of motifs like vampires and werewolves, but they just don't get as ugly as gothic literature. That means the big climax scenes in these series sometimes have "canon-atypical violence" because they're sharper than average. Diminished Expectations is postwar gloom. It's just ugly in general, although some individual entries are brighter. So if I said "canon-typical" there, it would be quite a lot nastier, because the baseline is already among the darkest stuff I write. The C-T and C-At warnings invite readers to think about the overall tone of the series. If you keep ouching on entries from a certain series, consider moving a step or two lighter, because I have other ones that aren't so intense.
#11. CNtW violates that trust readers place in authors.
This one is interesting. First, trust is typically based on knowledge -- either an individual or a group that you believe will act in your best interests. If you don't know someone, you probably shouldn't trust them. You should exercise some caution until you know them better.
This is very important for readers and writers. As you get to know each other, you learn what works and what doesn't. I've learned that my readers will trust me to lead them into some extremely dark alleys, because they believe the payoff will be worth it, and I try to deliver that. They've paid enough attention to know what I write and that they like it. People new to my work don't usually hit the darkest stuff until after they've read enough of the lighter stuff to decide that I know what I'm doing and will not drop them on their heads.
Regarding the contract between writer and reader, the most universal is: Thou Shalt Not Bore Thy Reader. The whole point of publishing is to entertain people. (It's okay to write for other reasons, in the privacy of your diary, but once you engage other people, they deserve to get something for their time.) Fail this standard and nobody will be happy with the story. Pretty much everyone agrees on this as a standard.
Beyond that it diverges. People generally expect a story to be "good" but don't all agree on what that means. There actually are a lot of technical, objective factors (e.g. correct spelling) but having or not having those isn't 100% accurate in identifying good or bad literature. People generally expect a story to avoid injuring them or doing other gratuitous damage, but again, they don't agree on what that means. Some people are damaged where others are amused or enlightened. There are trends, but no absolutes here either.
That said, you should try to write what you think is good, and if you choose to write dire things, it is polite to let people know that in some fashion. While books don't have content warnings, they do have genres and cover art and titles that give you clues about the material. You don't go to the Horror section for fluff.
I like when my fans trust me. But I don't think strangers should trust me beyond Thou Shalt Not Bore Thy Reader. How could they? They don't know me yet.
#12. CNtW sends a message to trauma survivors that they’re not welcome in fandom and springs 'harmful surprises’ on them.
I defend that tag partly because of survivors I know. People who might not always have the extra spoons to make complete tags. Stories that meet the letter of "no archive warnings apply but violate the spirit vigorously and without a condom. That sort of thing.
#16. There’s no reason authors who CNtW can’t just put their warnings in an endnote.
To lack of time/energy I will add lack of ability. Accurate tagging isn't easy, people go apeshit if you get it wrong, and that's a miserable situation for anyone with anxiety or imposter syndrome.
Furthermore, some people write fanfic purely for fun, and tagging isn't fun, it's work. They don't have to do work if they don't want to. That's the beauty of fanfic over professional writing. In fanfic you can say "fuck it" and do whatever you want, including skip the things you don't find fun.
#19. Fanfic isn’t worth hurting real people.
Now if only the gripers felt that way about the folks whose opinions differ from their own.