The Hobbit seems like kind of this weird gray area.
For starters, it's based on a nearly century old literary classic. Then there's the fact that it's part of a hugely popular film series which has already given us much of the conclusion to the story. Not all of it, obviously, but we do know where the most prominent characters end up after the adventure, due to the Lord of the Rings. It's similar to whether in the Star Wars prequels we should tag the fact that Anakin fathers Luke and Leia, or Obi Wan ends up on Tatooine.
I'm not saying it shouldn't be spoiler tagged. But I think it's difficult to decide what needs to be spoilered. And spoilering everything limits the discussion from those who would like to partake in conversations about the film but don't want to risk having major plot points spoiled. Someone who wouldn't mind "I love the fight in [insert place]," but who doesn't want to know "I can't believe that Gandalf is [a cyborg sent from the future]. What a twist."
But yeah. Spoiling anything from an unreleased film that hasn't been included in promotional material is cut-and-dry bannable shit. I'm glad I missed the Pacific Rim post.