There are some writers who think everything that isn't in the rules is opt-out. So if there's anything that will actually truly mess you up in the "panic attacks and weeks of nightmares" sense, not just the "I got a story I didn't like much" sense, DNW it no matter what. You'll never cover enough DNWs for all the things those people might think is okay though, so I wouldn't worry about it beyond those really, really important ones.
Also, anything that's a major part of canon is opt-out. Normally, say, incest is opt-in, but if your fandom is Flowers in the Attic, you need to explicitly opt-out.
Also, if you're the kind of person who does like most things and puts a note in your letter along the lines of "I am pretty much squick-free, go for it!" then you are saying that everything is now opt-out, so opt-out of anything.
Similarly, sometimes people who are wondering if something they are thinking about is okay will sometimes look at your fic/other gifts/recs etc. for the fandom you matched on, so if you have just rec'd a bunch of 24/7 power exchange fic on the same account as your letter, but don't want it for YT, you might want to mention that.
Other than what's covered there - In Yuletide, anything NC-17 is probably opt-in. If you don't mention wanting sex in your letter you probably won't get anything explicit. (This doesn't mean no daddy kink though, sadly.)
Everything that requires an AO3 archive warning is definitely opt-in, or should be.
Most people won't write you "hard kink" unless they have reason to believe you want it. The problem appears when people disagree about what "hard kink" is because it's so mainstream in their corner they don't realize it isn't in other people's. Nearly everybody agrees that, say, scat or collaring or bestiality or tentacles or sibling incest are opt-in (unless they're canon) so those are probably less important to DNW honestly.
Things I'd say the 'not everybody agrees on the kink level' applies to are things like: daddy/mommy/little kink choking/breathplay spanking/discipline a/b/o (m)pregnancy low-key genderplay kink various levels of arguable dubcon, like drunk sex or secretary/boss incest between cousins/stepsiblings/foster siblings consensual nonmonogamy blood fuzzy-handcuffs-level bondage
Probably some others that could be added to the list.
Re: Kinks that are opt in/opt out
Also, anything that's a major part of canon is opt-out. Normally, say, incest is opt-in, but if your fandom is Flowers in the Attic, you need to explicitly opt-out.
Also, if you're the kind of person who does like most things and puts a note in your letter along the lines of "I am pretty much squick-free, go for it!" then you are saying that everything is now opt-out, so opt-out of anything.
Similarly, sometimes people who are wondering if something they are thinking about is okay will sometimes look at your fic/other gifts/recs etc. for the fandom you matched on, so if you have just rec'd a bunch of 24/7 power exchange fic on the same account as your letter, but don't want it for YT, you might want to mention that.
Other than what's covered there - In Yuletide, anything NC-17 is probably opt-in. If you don't mention wanting sex in your letter you probably won't get anything explicit. (This doesn't mean no daddy kink though, sadly.)
Everything that requires an AO3 archive warning is definitely opt-in, or should be.
Most people won't write you "hard kink" unless they have reason to believe you want it. The problem appears when people disagree about what "hard kink" is because it's so mainstream in their corner they don't realize it isn't in other people's. Nearly everybody agrees that, say, scat or collaring or bestiality or tentacles or sibling incest are opt-in (unless they're canon) so those are probably less important to DNW honestly.
Things I'd say the 'not everybody agrees on the kink level' applies to are things like:
daddy/mommy/little kink
choking/breathplay
spanking/discipline
a/b/o
(m)pregnancy
low-key genderplay kink
various levels of arguable dubcon, like drunk sex or secretary/boss
incest between cousins/stepsiblings/foster siblings
consensual nonmonogamy
blood
fuzzy-handcuffs-level bondage
Probably some others that could be added to the list.