Few things can get me more hyped about a game than its trailer. Hopefully they do more than just show cutscenes and gameplay. Hopefully they can give you and feel for the game and its story. Like a movie trailer would. Nothing has done a better job than the launch trailer for Mass Effect 2, imho. From Illusive Man's narration, to the scenes, Shepard's speech at the end and the music - oh that music. Truly epic!
I'd love to hear what other trailers people thought are great. I love watching them - especially on a slow Friday at work ;)