I think the biggest thing those editing trailers do is focus too much on plot. You'll finish the trailer and feel like you've watched the entire movie. The editors seem to think that if you don't know the entire movie, you won't want to see it. That however, is completely the opposite of how it should be. Let's go back to the 10 Cloverfield Lane trailer.
|Bad Robot loves greenish cloudy skies and a silhouette|
We open on a trio of people doing various activities to an upbeat score. However, the music starts to slow as we are shown images that this situation is less than ideal. The trailer ends with the female seeing something horrible but we of course do not know what she sees. All we are able to gather from this trailer is she is in the bunker and it's less than ideal. Everything surrounding it is left as a mystery. However, later trailers ended up showing far too much and revealing most of the mystery around the film, this one however does a great job. Bad Robot is well known for having well done trailer built around lots of mystery. Look at their trailers for Cloverfield and Super 8. However the latter did suffer from being marketed wrong. What we assumed would be another Cloverfield-esque horror film ended up being a coming up age film with a very Spielbergian feel.
The trailer makes the film look like a straight out horror film. However Del Toro was explicit in pointing out that movie was more of a "Gothic Romance". However, the trailers didn't reflect that and many people went into it expecting much more of a scare and were treated something that was absolutely beautiful, but not up to their par in the scare department.
I'll end by talking about what I think is a near perfect trailer. Star Wars: The Force Awakens.
While nearly every trailer for this film was fantastic I think this one stands out among the rest. With only eight short clips and a minor bit dialogue, the trailer perfectly captured the spirit of the film without giving any story away. Never before has a trailer satisfied me over a year before the film.
Be sure to also check out this list of trailers for great movies that are pretty awful.