On the topic of spoilers, I've come to realize that it's not really even about the plot. When it comes to films, it's those memorable moments that often get spoiled by people.
In my giddiness after seeing Inglorious Basterds, I posted about the gory death of a certain fascist dictator in a thread (a surprising deaths thread, I think). Someone who was rightfully pissed off by this rightfully took me to task for it.
I still feel like shit about it. I took away from someone's first experience of seeing Basterds. It might not ruin the film (since it's so great!), but it's just one of the film's sweetest pleasures. I'd have been pissed if someone spoiled that for me. Since then, I'm tried my best to be sensitive about that.
Another example: the gunbarrel opening of Casino Royale
Phil will back me up on this one. I had NO idea moment was coming and seeing it play for the first time was a legitimate thrill. If someone had told me about it, I doubt it would've had as much impact. Nothing to do with the story/plot, obviously. It's just that first time experience.
EDIT: And I just realized that I repeated everything I've already said on this thread last year. Dammit!