Talking about games is always interesting, while games are unique in that the "player experience" is the core of player interaction, the idea of surprise and the unknown is still a heavy motivator and part of the experience. Telling someone who've never played Super Mario Bros that there are 8 castles, and that every time Bowser appears will end with a ruse will ruin the experience. "The princess is in another castle" was such a minor moment in hindsight, but an important one that should not be spoiled.
With this in mind though, It's hard to have a meaningful discussion without spoiling something. For example, discussing something like the story for Braid or Portal would be impossible for someone who hasn't finished it without spoiling the game. So, I'm going to establish 2 rules:
1) Statute of limitations - 10 years. If the game's been around for 10 years, it's open season. Aeris dies. Get over it. If it's been 10 years and you still haven't gotten to it, then you're late anyways. Feel free to spoil something for me that's been more than 10 years too.
2) If it's recent, I'll always precede the post with ample warning about potential spoilers. Do yourself a favour, hit the back button, and come back once you've played the game in discussion.