Spoilers. One of the worst thing in the world, it sucks the fun out of most things that can entertain you including books, movies and yes, games.
Thing is, for the most part people say that all spoilers are story related. But while this rings mostly true for books and movies, I find it's far easier to 'spoil' games. And by that I mean the element of surprise.
A recent example: Diablo 3's expansion #Reaperofsouls. While the vanilla game had an ok story that was flawed in its execution, the twist with Leia was one I hadn't forse
The story in RoS was pretty disappointing. The first game many complained was bad and in hindsight I thought so too but a few things aside I enjoyed the ride. Some jerk did spoil a major twist in the Youtube comments which damned most of the suspense the story had, but all the other things were alright.
In hindsight I wouldn't have cared much if anyone spoiled anything of the story because quite frankly the only interesting thing story wise happens at the opening cinematic: Malthael takes on the evil guy hat.
The spoilers that I care (more) for are the actual contents.
Monsters, boss fights, abilities and environments are a pretty big part of games. If I had seen pictures of every last zone in Super Mario World 3D then the whole element of surprise and discovery is taken away. It mutates into a waiting game, patiently waiting until you get to see what you've seen before, but be in control of it yourself this time. The magic is mostly gone.
These days trailers in movies but also preview articles for games really give away a lot about what they're showing off. Developers and/or publishers need to find a healthy balance of showing off what they need to for folks to get interested in their game, but assure that there are still other mechanics and whatnot left to discover. I think Assassin's Creed IV and inFamous Second Son did this quite well.