In lieu of The AV Club's recent list of trailers that function as art (and, in some cases, are superior to the films they advertise), I offer you the most disappointing trailer of all time: The Omen (2006).
I still remember my cautious excitement upon seeing it in the theater. The simplicity, the lack of narration, the reliance on a dread-inducing squeaky swing, and, of course, the delightfully silly reference to 666, set me up to believe that the world had a chance to see a remake superior to the rather ham-fisted original--overcoming my inherent skepticism of remakes.
Alas, we got Liev Schreiber in mope-mode, and a movie that plummeted from the rooftops of possibility to a lawn of mediocrity. (I tend to think that Damien would not approve.)
It's been four years, and I'm still angry about it.
Added: So of course my feelings are in a blender about this.