You’re a sucker if you expect the studio system to churn out movies that will be great. Trailers are edited by a few highly paid and very experienced companies in Hollywood that aren’t so much, as Dan says, trying to get you interested in the story as they are trying to get you to pay and get in the door.
There are a lot of different routes that trailers can take to get you interested – from condensing the plot down to two minutes to just showing you a bunch of random scenes that seem exciting. Some go for the VO take, some go for telling the story with text, but without fail, if a hollywood studio is distributing the film, they are not trying to reflect the quality of the movie, merely trying to make it seem worth paying for.
Trusting a trailer to convince you to see a movie is like taking a con man’s word that he is not conning you.