I will give you fair warning now, this article will contain plenty of spoilers. After all, in order to talk about and discuss spoilers said spoilers actually need to be talked about. So, be aware of this. I will try to keep it to only well-known examples and/or only minor things, but what each person has seen and considers to be a spoiler varies, so be warned.
Spoilers are a tricky beast to pin down. In concept it sounds amazingly simple. What is a spoiler? A spoiler is a plot twist, reveal, climax, or whatever else that drastically alters how the story has been unfolding. Some of the most classic involve the reveal that Vader is, in fact, Luke's father. This drastically alters the relationship between the two characters, for obvious reasons, along with the grand story of the Star Wars series. Without it, the entire story changes drastically. Revealing that this is, in fact, the case, then, can change how someone watching the movies for the first time will understand the entire franchise. In games, a classic example would be the death of Aerith/Aeris, which sends players through an emotional mood whiplash. It's a doozy the first time this happens, as audience is not expecting it and it can elicit a massive response from them. Shouldn't it be obvious, then?
Well, not exactly…
You see, for the longest time, spoilers simply weren't a thing. Warp back in time to the 1900s and reveal the outcome of the newest Sherlock Holmes book and you wouldn't hear a peep out of anyone about spoilers. They simply didn't exist. For the majority of history they didn't exist, and it wasn't until the release of Psycho, in which Alfred Hitchcock asked the audience to not spoil the ending for their friends, that the term even existed. In short, spoilers were an advertising campaign.
The idea still holds merit, though, right? That certain story elements shouldn't be revealed for the enjoyment of others? Well… here's the next problem: this thing called 'tropes.' Tropes are elements of a story. Sometimes they can be considered clichés, but this isn't always the case, by a longshot. Some basic, game-related ones are the idea of checkpoints before certain sequences, signifying a boss, a weapon disproportionately strong for the section of the game it's in (be it developer design or accident), the classic 'Towers of Hanoi' mini-game that pops up in some form in the majority of Bioware titles, and so on, and so forth. In regards to story, there are some very classic and iconic ones; for example, the 'Red Shirt.'
This is a reference to Star Trek,in which whenever someone needed to die in order to show just how deadly/serious/aggressive the new alien of the week was, it couldn't be one of the main cast because, well, they were the main cast, so it would fall upon someone wearing a red shirt, which designated them as a member of the security forces, to be devoured/shot/impaled/whatever by the alien. Wearing a red shirt and not being a main cast member was practically slapping a 'kill me first' sign on your back. In horror movies, among the first to die would always be a couple that had snuck off to engage in romantic actives, especially if they were kissing and removing clothes; the question would not be 'will the monster get them?' but 'how long and will it be cool?' Saying you have two or three days until retirement, that this is your last mission, that you're looking forward to returning home to your spouse and kids, and being the best friend of the main character who just so happened to be introduced a scene before, practically guarantees that the criminals/aliens/whatever will kill you swiftly… and brutally. In both X-Com and Fire Emblem releases, thinking a battle is won because there is just one measly foe left who seems to be too weak to do any real harm and couldn't possibly kill one of your team is a sure-fire way to ensure that they target the one guy who just so happened to be wearing red while making out with his love interest because, wouldn't you just know it, he's only two days away from retirement!
If you're well aware of tropes, then certain things will be obvious, right? While you may not be able to figure out that it's Aerith/Aeris who dies, surely it's not a spoiler to reveal that all of Avalance, who just so happen to be wearing red no less, is going to die, because knowing the tropes tells you it would be a surprise if they weren't killed/pushed into the background. Well…
See, here's where it starts to get dicey. Not everyone is trope-aware, so something that seems like a no-brainer to someone versed in the ways of the trope may be a huge spoiler to someone who isn't. Likewise, someone who is may not even realise something is a spoiler in the first place because it's just that obvious. Is Mario going to find Peach in the first castle? Hahahaha! No! Otherwise there wouldn't be seven more worlds to go through! Wait, that's a spoiler? How so?
Of course, this isn't just a one-sided thing. Some people get very defensive about spoilers to simply unreasonable degrees. Talk about the machine beasts in Horizon: Zero Dawn? 'Dude! How could you spoil that for me!' …even though it's on the cover of the game box. Talk about the weapon breaking mechanic in Breath of the Wild? Spoilers! Talk about how, if you engage in an insanely difficult and unlikely chain of events, you can end up with a horse as king in Crusader Kings II? Spoiler! Reveal that the Night Elves and Undead are playable in Warcraft III? Despite having come out years ago and having a successful MMO based off it, some people would claim that that is a spoiler. Way to go Ofisil! Your review of Warcraft III spoiled the game for me… because of something you couldn't have possibly anticipated and was perfectly acceptable for you to discuss, yet my hyper-oversensitivity to the notion of spoilers has caused me to have my experience with the game ruined! (Note: I've already played Warcraft III and am fully aware of all its plot and story and in no way consider what Ofisil wrote to be a spoiler. This comment is for satirical purposes.)
While some people may be doing this just to hush up an annoying person who just can't take a hint that they don't want to hear about whatever it is anymore, some other people are genuinely offended. My personal philosophy is that if something happens in the first one to three hours (depending on game length), or is important to the plot getting underway, or is obviously spoiled by easily accessible outside information (for instance, playing Street Fighter IV reveals that Ryu doesn't die in Street Fighter III or that the French/Franks will be playable in Age of Empires: The Age of Kings, which is a medieval-era RTS with real life cultures/nations involved), then it isn't a spoiler and the person is over-reacting.
This leads to the crux of the whole article. If a show/movie/game/whatever becomes boring, uninteresting, bad, or whatever, to the point where you don't want to go for a second round of it after you know the spoilers, is it actually good? Is it a spoiler to reveal to your friend whom has seen GoldenEye a plot-point that happens in both the movie and game because he already watched the movie? It's not an easy question to answer.