Warnings are important. Very, very important.
And the more I read about how important they are, the more weak the arguments against warnings become. It's not about "ruining the surprise" of the fic, it's about protecting readers from being squicked and/or triggered by the fic. For example, as of right now, I cannot read fic that deals with terminal illness. Can. Not. (ETA: At least, when one of the main characters is suffering from one.) If I was reading along and enjoying a fic, and then suddenly encountered one of the characters getting a terminal illness, I would have to stop reading. I wouldn't be triggered by it per se, but I would be reminded of my father's battle with cancer and thus be unable to enjoy the fic. I'd also feel a little betrayed by the author because of their lack of warning, and I'd be wary of reading their other works.
Like I said, warning for content (or even just putting a "this fic contains" list to avoid the connotations of the word "warning") is not about ruining the author's ability to create suspense and surprise in their fic. I hate spoilers, and I wouldn't want big plot points revealed before their time. One thing that does bother me, but that I wouldn't want to know about beforehand would be anything to do with animal abuse/animal death. If I knew about it, I would be constantly waiting for it, and I wouldn't be able to concentrate on the rest of the fic. However, if it came as a surprise, then I wouldn't be fixated on it, and the emotional impact would have a greater effect.
Now this is where I'm going to sound like a bit of a hypocrite, but bear with me. I'd still want the animal death to be warned for. I'd still want that option available for people who might not be as comfortable reading about it as I am. And there are ways to warn for things without spoiling them for people who would otherwise be fine. For instance, you (general you) could use the highlight-to-read method, which I posted about
here (warning for discussion of warn-worthy subjects: sexual assault and the aforementioned animal abuse). You could use the caveat lector method, which lets readers know that there's something there that's squicky/triggery. You could even make a separate warnings chapter, which also lets readers know that there are things in your fic that deserve warnings, but it gives them the choice whether to read those warnings or not.
Personally, I tend to be as specific as possible with my warnings, using a combination of general warnings with more detailed highlight-to-read ones. I'd hate to make readers uncomfortable or triggered by my fic, so I try to be as upfront as possible about the contents. I try to treat my readers the way I'd like to be treated as a reader.
Peace y'all.