In certain circumstances, spoilers change the nature of a story, I don't think however, that they actually spoil it. Not for me anyway. Say, for example, The Usual Suspects, the Kevin Spacey film. Brilliant film, and the first time you see it, without knowing the ending, watching it unfold is a brilliant thing. Knowing the ending doesn't detract from the experience though, as I've watched it more than once and it's still a good film. Knowing the ending makes it different, not bad. There are however, very few instances where this is the case, i.e that knowing the ending will significantly detract from the story. Especially when you get to films, as 98% of films that you watch are going to have a ye olde classic ending. Most of the fun is in how the characters get to the end of the story. And that, quite often gets ways to complex to explain, so spoilers aren't a problem. Try explaining the end of Scott Pilgrim vs the World, or Kick Ass. It can be done, but most descriptions are going to leave you flat - going and watching the endings after they've been explained is still a joy.