The following contains spoilers. Duh.
I probably spend more time thinking about how much to "spoil" about a movie's plot than I do about any other aspect of writing film criticism; mostly because it's the only aspect that doesn't come naturally. When studying film academically and writing about it professionally, it's fairly common that a lot of one's immediate social circle begins to include folks on the same beat; and a common thread that emerges among said folks is that you've all either A. already seen anything that might be under discussion or B. don't really care all that much about plot details as a major part of your cinematic experience. I can't stress the reality of "B" enough - for a lot of critics, actually being "involved" in the narrative on an emotional level (as opposed to analyzing how the beats and pacing function mechanically) is the equivalent of watching pro-wrestling and actually believing that the geopolitical realities of Iranian/U.S. relations would be effected by Sgt. Slaughter's battle with The Iron Sheik.
Long story short, when critics talk amongst themselves, spoilers flow freely. So too do they in publications and websites understood to be mainly written-for and read-by would-be film academics. But in "general audience" reviews it's always been considered verboten to "spoil" the surprises of a given film -- after all, that's part of what people are paying to see. Trouble is, it's gotten fairly difficult to gauge what actually constitutes "spoiling."
Once upon a time, this was easy: Most mainstream films adhered so rigidly to formula that any "shocking" deviation was easy to spot. But today, so many movies are constructed as self-aware puzzle boxes to one degree or another that people are bound to cry "foul!" at any suggestion of plot detail beyond a basic cast list and a ticking-off of who the heroes and villains are.
It gets even worse when it comes to the current wave of films adapted/remade/rebooted/etc from existing material. You'd think it would be easy to spot the "do not spoil" signs in, say, a superhero movie: Anything that's a surprise to the hero is probably meant to be a surprise to the audience, ditto the now-customary post-credits stinger. But I saw folks complaining about critics who mentioned in passing that the Batroc the Leaper turns up at the start of Captain America: The Winter Soldier. Mind you, Batroc has been in the film's cast listing forever, just not prominently (mostly because nobody cares about Batroc the Leaper) and his appearance isn't so much a surprise as it is a minor bit of worldbuilding detail for devout Marvel fans.
But ground-zero for "spoilerphobia" is in TV criticism, which is a literal 180 from where it used to be. Not even a decade ago, it was widely accepted that it was perfectly okay to talk about ANYTHING from a given show immediately after its first broadcast, as that would be when 98% (at least) of anyone who cared would've been watching. Sure, people would record stuff via VCR (ask your grandparents) if they weren't going to be around, but no one would've considered it any kind of major social faux-pas to speak openly about the ending of some major TV event on the off chance that someone in the vicinity had yet to see it. In fact, it would've been more rude to complain about such things -- "Hey pal, who are you to tell me what I can and can't talk about in public?"
Not so today, in an age of DVR backlogs and wait-for-Netflix bingers. I'm pretty sure that if I was to appear in court charged with strangling a recently-returned one-legged Iraq War veteran and father of six to death with a bike-chain and offer as my defense "He was about to talk about last night's Game of Thrones and I'm still only up to season 2!;" the jury would not only find me not guilty but order his surviving family members to pay me punitive damages.