Film editing is a hobby of mine, but anyone can clearly see that there is an art to the movie trailer. Obviously, they're a mixed bag. When poorly done, a trailer will spoonfeed you the entire narrative arc of the story. They often give away the 3rd act and show you the best shots. Comedy trailers are notorious for revealing all the best gags. Most trailers are generic and repetitive, one indistinguishable from the next. However, when expertly made, a trailer will capture the mood of the film through sound and image, only using moments of the plot to hint at the larger story. The best trailers provoke an emotional response and work as stand-alone short films in themselves. A bit hard to explain so let me find some recent examples...
Garden State: maybe the best trailer from the last couple years. And it is also a good example of a trailer that ended up much, much better than the film itself.
Requiem for a Dream: a fantastic trailer which skillfully captures the mood and style of the feature-length version. Still scares the shit out of me.
Jarhead: another good example of a trailer that far exceeds the film. The actual movie is boring in comparison.
Marie Antoinette: more a teaser than a fullblown trailer, but the use of unexpected pop music (New Order) makes this a standout.
Elephant: another perfect use of music to much different effect.
Syriana: in this instance, the trailer makes more sense out of the plot than the actual movie.
Kill Bill: an example of a great action movie trailer and yet again, awesome music.
Pearl Harbor: remove all memory of the actual film from your mind when watching this one. Yes, the film sucked but this trailer is amazingly effective and packs much more of an emotional wallop than the film ever did.