I think a truly good piece of media is one where even if you have things spoiled you can still get a reaction. All things considered the main character pretty much always wins/lives and the main villain pretty much always loses/dies. And one I've noticed a lot is that the first even possible romantic interest is most likely who they'll end up with if they end up with anyone. It's not foolproof but I'd put a bit of money on it. It's the in between that matters.

To go a step further, sure he killed Dumbledore but without knowing why, and without knowing the real why that's revealed later, only a small part of the fun is lost. That's what makes it re-watchable to me. I'll remember the twists and the endings, but the emotions and the minutae are what keep me watching.