Dang you got me rambling. Detailed thoughts on tagging vs no tagging under the cut because for me it's such an interesting topic of debate.
Oh man this is such an interesting problem for me as well, because I also can't decide which team I ultimately land on. I just get it either way, and I guess it depends entirely on how you approach reading and ultimately why you might want to read in any given moment.
ff.net is like picking up a book at the library, which people have done for centuries. Look at it, maybe you know the author, maybe you like the blurb, maybe your friend told you about it. You read it, and maybe you're surprised! I mean, you likely will, it's a new book, but if it's a pleasant or unpleasant surprise is anyone's guess, ultimately. And if you don't like it, you shut the book, put it down, and walk away.
AO3 is the ultimate curated experience. You can find exactly what you want and you can avoid exactly what you hate. It's unlike any reading experience from ever before: you'll get your favorite story dumped right into your lap most likely, with tags showing it might as well have been designed for you. You'll probably like it! Statistically you're bound to. And all those things you hate are left to the side. But what about the element of surprise? You know it's a slow burn, you know someone might die, you know pretty much exactly how it's going to go before you even find it, because it's exactly what you were looking for the whole time.
(Obviously there's nuance to both: some authors on ff.net issue some warnings. Some authors on AO3 undertag or overtag. This is a generalization.)
Neither of these is bad but as someone who publishes online it's such a gamble, and part of me likes...both. Either, depending. I don't want to spoil the story, I don't want to show my hand, but I also want to make sure people are happy when they find my works, and I want them to be able to find my works in the first place! There's no way to have the best of both worlds unfortunately, not that I can think of. Maybe a switch on AO3 to hide the more specific tags (or just...don't read them), showing only major warnings, rating, and the summary? I don't know. Because as you said, sometimes you might find your favorite fic not because you sought it out explicitly with the tags, but by a pleasant surprise! That unassuming book you never heard of could be your favorite one ever! And that's amazing! Then again, enjoying a read only to get that unpleasant plot twist?
I like both. I can't agree on either side. Part of me does suspect that the tagging might have made readers a little soft, and I don't mean that in an unkind way: I just think it's an important life skill to be able to cope with reads you don't necessarily like, characters and decisions you disagree with, story developments that are unpleasant, either by being able to digest it, analyzing it, or just by having the strength to walk away.
Ultimately, I guess it depends on why you're reading: do you want to have fun, guaranteed, on your tailor-made experience? Or do you want to be taken for a ride who-knows-where? That, ultimately, decides which one might be better for the given reader, or at a given time. If I'm hankering for something specific, I'll definitely sift through those tags like nothing else. But sometimes I just find something with a long word count, and fling myself off the deep end. Gotta have a little bit of both at least, I think.