One of the creators of advertising rules explains that for an advert to work properly, you should not or at least not completely reveal its object, in order to arouse the spectators’ curiosity and make them want to see more.
Hitchcock knew this when he used the same principle to inspire fear.

The authors of Rec, a Spanish horror movie, also use this theory to promote their teaser. It has been available for a while now and has been posted on quite a number of blogs already. It does not show the film, only its spectators. Impressive, to say the least.

[REC.] Trailer and audience reaction
Uploaded by BriKO

Still, it is not yet possible to say if we are dealing with a new masterpiece or another stupid, gory horror movie.
The story at least is insignificant: A Barcelona-based unit of fire-fighters find themselves shut into a strange building after having gone there following to call for help…

We will have to wait until April 2008 to know more.