I don't care too much for story in video games; it certainly wouldn't be the focus, if at all present, in my ideal game. Given this, therefore the sorts of games I do play, there's nothing significant to spoil so I don't mind. I think developers should find more gameplay-driven ways to compel players to carry on; if I wanted a story, would I not read a book? The nice thing about free-form gameplay is that it can only be spoiled by the player getting too good, hence another penchant of mine is for games with steep learning curves.
Anyway, I'm going a bit beyond the scope of your question. At the end of it all, I would rather not have plot-points spoiled, but I don't ultimately care either way. Odds are the protagonist wins, maybe dying thereafter. It's nice though when things are affected by the player's actions, as in the Mass Effect series, and those are the sorts of games I'd rather not have spoiled; but again, if I do know what will happen while I'm enjoying it anyway, such knowledge doesn't kill a game for me.
As for me spoiling things for others, well, working on the principle of 'treat others how you wish to be treated', I may or may not go ahead if the subject comes up, suppose it depends on the exact detail. If I do, it's with no malicious intent, I just don't think it matters too much, so don't presume it so with others.
