As others have said, the definition of what is considered a spoiler or not will be in the eye of the beholder and can be judged on a case-by-case basis. What I may perceive a spoiler to be could be trivial information to someone else. I prefer to know as little as possible, since that way, I don’t go into something with preconceived notions of the story, characters or themes. I define a spoiler as: “Communicating or discussing a plot point, event, scene, context or tonal shift regarding a piece of entertainment media (anime in this case) that couldn’t be inferred from a spoiler-free summary.”
What I mean by communication can vary. It can be discussed in a forum, word of mouth from a friend, a YouTube video or even an image from a search. Plot points, events, and scenes are pretty self-explanatory since if I know what is going to happen later in the narrative, the surprise is gone and I expect it to happen. However, I would also consider context itself to be a spoiler. It is one thing to know the what but the why is even more spoiler-y in my eyes. Even though I still think the events in a story are spoilers themselves, I do think it is vital that people don’t know about the events that lead up to it or how it that event changes the story and the context with how the art is looked at.
An example I will use is Madoka Magica (major spoilers ahead for those who haven’t watched it): I personally think knowing how Mami’s death in Episode 3 changes the story, themes, and especially tone of Madoka Magica is even more of a spoiler than the event itself. The show at first is presented like your average, lighthearted Mahou shoujo anime but once Episode 3 hits, everything tonally shifts, shattering the viewer’s idea of what Madoka was and transforms into something completely different. I have been guilty of this several times, but saying that Madoka Magica is really dark to someone who doesn’t know anything about the show (nor has seen a single episode) could potentially be a huge spoiler in itself since the first two episodes (from what I understood) are set up in a way that you wouldn’t necessarily suspect that a drastic tonal shift is coming. Knowing Mami dies is bad enough, but also knowing that Madoka Magica is dark because of that (or in general) is even worse, since it hampers the impact the anime can have on an unsuspecting viewer.
While I do like going in as blind as possible, there are specific things I do not see as spoilers necessarily. Given a new Shounen anime called A is coming out, saying that: “The new anime A has people who fight” is not a spoiler, since it is fairly obvious that those things would be true given the genre of manga/anime it is in. Another thing I don’t really consider to be spoilers is grades or ratings that an anime gets. A score is purely subjective but the reasoning for that score can become spoilery, depending on how in-depth the review is. What is considered “good” or “bad” is purely on the individual and isn’t an objective fact, unlike an event that happens in a story. Furthermore, like others mentioned, I feel that when viewing or critiquing an anime or any piece or media, we need to be able to see through the hype that it gets and judge it on what it is to us and when possible to not let outside influence affect our critique (though I recognize that can be tough). What I personally find really a gray area in the subject of spoilers is trailers or promotional material. Unlike a summary, trailers I find more often than not spoil the events of the story or give some context to it. While a good portion of anime trailers are in Japanese and a good portion of Western audiences aren’t fluent or can’t speak Japanese, there are still dubbed trailers that can spoil events or contexts early on that you wouldn’t know from the description. Promotional materials in the form of advertisements have spoiled entire twists in a story, which can hamper the experience for a lot of people. Not all trailers nor promotional materials do this, but I still avoid them as much as possible, since I rather judge something with completely fresh eyes.
Those are just my two cents on the matter.