Allow me to be the first to say SPORE.
Good Lord, that game had me hyped. I was on the edge of my seat, drooling for it, checking every day in case the release date somehow changed.
And then... it was a handful of mini-games strung together.
The creature creator was great, everything it was hyped up to be (I thought), but there was just nothing to go with it. Except the space stage, which was the same thing over and over again.
I don't think I've ever been as disappointed by a game as I was by Spore.