For an object lesson in how editing and music in trailers can make enormous differences in setting expectations, take a look at the clever mock trailers for THE SHINING and WEST SIDE STORY.
THE SHINING has been transformed into a heartwarming domestic comedy while WEST SIDE STORY becomes a horror film. The former is a brilliant piece of work. If I didn't know any better, I wouldn't suspect that it completely perverts what the film is actually like. The rejiggered WEST SIDE STORY trailer is pretty good too, although it "cheats" in adding some well-placed special effects to enhance the illusion.
(Credit to Rob Blackwelder for posting these links on the Online Film Critics Society forum)
Addendum: I found another one linked on the P.S. 260 blog. It applies the horror film template to TITANIC. Not bad but not as inspired as the other two.