I don't mean the ones you see at the theater; I'm talking about those used as advertisements on the internet and stuff. More and more often I find myself agreeing with the commenters who make jokes like "what's the point of seeing the movie now; I just saw half of it in the trailer."
And I don't normally see this with kids' movies, it's live action dramas (and even dramedies) that seem to have this problem...but maybe it's not length; just substance. I mean, five minutes worth of an actual movie isn't actually that much; it's only ~1/18th of the full thing. In an action/adventure movie, you could easily fill that with chase scenes and scenic shots/voiceovers, and then maybe one or two character highlights. It's not hard to avoid spoiling the entire film.
But with dramas, where the whole thing is just character highlights, five minutes worth of footage can easily spell out every arc in the story if you're not careful. I mean, have you ever had to give a five-minute speech? Dialogue is fast; you can say a LOT in that kind of time. Maybe even too much...
(BTW, I use 5 minutes as an example of a trailer that's already too long. I really think 2-3 minutes max is more than enough...)