The best thing about a good trailer is that it teases, leaving just enough spaces for viewers to project their own stories into the cracks.
Take Terrence Malick's The Tree of Life, for instance.
What the hell is it about?
It doesn't look quite like science fiction, despite the images of planets and stars. It might be someone's memoir, but it doesn't look like it's told in a straightforward way. We know just enough about it to wet our appetites and prime the storytelling function in our brains--viewers already begin to cobble together a story at the juxtaposition of Brad Pitt and a baby's feet--but our information is limited.
The genre is unclear. Surreal, maybe? The main character is obvious, but we don't yet know which age the story will follow him through, and thus we're unsure which actor we'll be asked to identify with most.
Tree of Life is getting a lot of buzz for its obliqueness. Getting the haunting music out of my head has actually taken some work. (It's from Czech composer Bedřich Smetana, if you're interested.)
Trailers are made to hold just enough back, I guess. I can think of only a few that have done this so effectively, and the first that springs to mind is one of Pixar's films:
A delicious build-up aided by "The Office" from Terry Gilliam's Brazil.
And a concise introduction to the main character.
My DVD copy of the film WALL-E is somewhere in my room. I'm very fond of the story, the tone, and the wonderful use of science fiction. Still, the movie I own bares only a passing resemblance to the movie hinted at in its teaser.
The teaser becomes the narrative, a story more thoroughly realized and emotionally satisfied in the mind than on film.