One of the most important elements of making trailers, or for that matter marketing a book, movie or game is not to give away the whole plot. Many authors are excited about the nuance and complexity of their story and want everyone to instantly appreciate their genius. They may well deserve it, but you need to maintain the mystery and interest for someone to buy and read the book. Many of us have sat in a movie theater and realized that every good scene in a film was displayed in the trailer. That is called desperation. If you cant hook your audience with the beginning of your story, you really wont with the whole book. Advertising is just a way to pull a consumer in and make them want more. Don't blow your genius in one shot. Hook them in and let them enjoy discovering the rest. Thats what we do. Tell just enough to make them buy.