I agree!
I do watch Movie trailers but normally only the first one as after that they become too spoilery. I especially can't stand previews for upcoming TV show episodes, I mean you already know if you are gonna watch the show or not so why go into it knowing part of the resolution to the just finished ep you watched?
I personally wish TV OT threads would all just be discussion up to the latest aired episode only and ideas of what's going to be happening, instead of the black bar filled threads we end up with now after an episode has aired and people are desperate to discuss the previews. I mean once the show has aired you should feel safe going into the thread but you never know anymore with how terrible some people can be with their spoiler tags.