Here be spoilers, but only to things that happened thirty years ago, and vague references to things which contain twists.
I think the internet's reactions to the Red Wedding versus John Harrison is interesting, because it shows the difference in spoiler-etiquette between those who are in the habit of being in a fandom and those who are not.
I don't know at what point it becomes acceptable to stop being careful around spoilers, but it's longer than "twenty minutes after that book is published/episode airs/film premiere screens." Even if someone if watching Star Wars for the first time (if they are older than twelve or so), it should hardly count as a spoiler to mention that Vader is Luke and Leia's father. It's fair game to make a joke about soylent green being people. Everyone knows Norman Bates mother was dead the whole time, as was Bruce Willis. It is safe to mention any of these things in conversation without being a jerk.
My sister, who has read the A Song Of Ice And Fire books already, advised me a week before the Red Wedding to catch up on Game of Thrones in time for that episode, because something important would happen and the internet would spoil it for me if I didn't watch it straight away. I thought this was a bit unfair on the internet - I've been years behind on shows and avoided spoilers easily! - but it aired on Saturday, I watched it on Sunday, and a friend who hadn't seen it yet had it spoiled by Facebook and Twitter by Monday.
I think it's because Game of Thrones is so widely popular. A lot of the fanbase are people who aren't usually heavily invested in a series, don't know the spoiler etiquette, and so reacted loudly and in detail on Facebook. Anyone who has spent any time hanging around forums or fansites and reading fan theories and fics and accidentally discovering what slash is at eleven because the masthead made it seem like it was a synonym for 'spoof' and feeling terrible Catholic Guilt as a result* knows that you tag your damn spoilers. If it's a TV series, you tag them by series at the least, and episode really. I think that's why, even though Star Trek: Into Darkness has been out for a month now, the internet has been pretty decent about not revealing John Harrison's identity. Even though the reboot doesn't have much to do with the original, the kind of people who'd go to see it are largely the kind of people who've obsessed over at least one series in their lives, and so know what bad form spoilers are. I began watching both Doctor Who and Supernatural years after they started airing, and the only things I knew in advance were general plot points (Space! This guy will be a new guy after this series! Bobby! Dean and Castiel don't kiss but the internet thinks they should!), and things I spoiled for myself by opting to disregard spoiler warnings**. Obsessive fanpeople: they are very considerate in specific contexts.
In further fanperson sensibilities, I find the semi-meme of referring to "Jawn" in the Sherlock fandom interesting. It's presumably meant to indicate Sherlock's pronunciation of it, but in my head he pronounces it "John", and "Jawn" reads like a drawl, and so more American. If they were trying to show the clipped, British intonation I would go for "Jaun" (as in faun, not maus). "Jawn" is close to how my head pronounces "Jon" which for some reason seems more American-sounding to me than does "John." Regional dialects - they are fun to dissect!
*Based on a true story!
** I spoiled the Red Wedding for myself, I had John Harrison spoiled for me in a post which used his real name a few days after ST:ID came out, which was the more inconsiderate given that most posts at the time were making a point of referring to him only as "John Harrison".