Sometimes I think that Hollywood believes that we're stupid. Do they really believe they can put the whole movie into the trailer, and expect us to forget what we saw? Unfortunately for us, this can kill the suspense when we actually go to see the movie.
For example, consider the classic What About Bob? This annoyingly funny movie builds to great climax. Bob places explosives around the doctor's house, which no one expects to ever go off ... that is unless you happened to see the trailers. If you were one of the unfortunate, then you knew the house was going to explode the whole time. So those hilarious ten minutes building to the climax? ... All ruined, because you already saw what happens in the trailer!
And a more recent example? <beware ... minor spoilers ... you might want to skip this paragraph> Consider Oblivion ... a visually stunning movie, while a little light on plot. I avoided the previews like the plague, knowing that one day I'd watch the movie. And I'm glad I did. Most of the trailers reveal that Sally is evil (not just a lying bureaucrat, but plain evil). Seeing how this is a main plot twist, what are you, the viewer, supposed to do for the first hour of the movie before this is "revealed?" Suspense ruined!
And I hate movies where ALL the good parts are in the trailer. When it's over, it feels like none of the movie is fun.
Well, I suppose there is one use for a "good" trailer. Sometimes if it's a movie you know you'll never pay money to see, the trailer will let you watch the whole plot for free, and it's all over in a matter of minutes. I'll close with this trailer that gets my vote for Most Revealed Plot...