I probably should just save this idea for my Slate job application, but I think "spoilers" aren't nearly as bad as people make them out to be. I knew Macbeth dies in the end before I read the play, I knew that Troy falls because they stupidly let a wooden horse full of Greek soldiers into the city walls, and I knew that things weren't going to work out for Anna Karenina and Count Vronsky.
To a large extent, I'm with Matt. I don't watch Mad Men to see whether Don is going to choose Faye or Megan. I don't much care. I watch it to see human beings tie themselves into knots, as human beings do, and then try to untie themselves.
With that said, I think it's worth noting that there are different ways to experience a great piece of fiction. The Sixth Sense, The Intuitionist and The Others are all great, even when you know the ending. But when you don't know the ending, you're watching a different film; you're making different assumptions about characters, and keying on different things. Specifically, in the works I mentioned, you're actually walking with the protagonist as a kind of ghost, figuring it out with them, but unable to dispense your (often wrong) advice. Knowing the ending obviates that way of experiencing the work, and there really is no way to have that experience again.
What's happened now is that we're living in the era of "the twist." A lot of shows don't so much have you walking with characters, as they have you walking with cardboard cut-outs waiting to trip over the next plot device. Or maybe it was always that way. I haven't consumed enough narrative to know. But it is mildly annoying to see fans focusing exclusively on "what happens next" as opposed to, say, "who is this person."