I know exactly what you mean, and yes, I have been that disappointed about a video game after watching such a promising trailer. For me, one of the Assassin's Creed games was like that, although it wasn't the most recent one. Also, the newest (at least I think it's the newest) Final Fantasy for Xbox 360 was a letdown after watching the trailer and hearing rave reviews.
Not really ~ I don't usually buy games from watching a trailer of the game. I like to play them first. I usually will rent the games first or play them at a friends house and then buy. I have gotten games that I have rented and found them boring.
Trailers are so misleading. It doesn't matter whether it's for a movie or a video game. They are nothing but promotional. I always check out blogs, review sites, and forums to get information and honest feedback about games. Nevertheless, I wish that these trailers were more representative of the actual game.
I have done that a few times with games but many more times with movies, its one reason I never fully listen to a trailer to make me want to play or watch anything anymore. Burn me one, you know? Now there are some trailers that are just amazing but those now make me research before before I invest anything other than time into it.