Eh, I think it's good because it gets you all hyped and ready to play the new stuff when it's released. It's just like watching a movie trailer, they get you motivated to go see the film because it shows some parts that got you thinking, "OMG! Are they gonna be okay? What's going to happen next?!"
When it comes to the dungeons/trials, yeah, they show the mechanics, but we have to figure how it works once the content is released.
Basically, if you don't want to be spoiled, then don't watch the trailers. With this in mind, they don't reveal everything on what happens in the dungeons, trials, MSQ, or the extra content that is coming in the patch; some things are kept secret.