Have you ever played a demo that you absolutely loved, one that instantly convinced you to go out and play the real game ... which turned out to be a disappointment in comparison?
My poster-child example would definitely have to be Wreckless: The Yakuza Missions. The demo mission was simple in formula — destroy a bunch of Yakuza members' cars by plowing into them with your own car or forcing them into walls — but it was simply a joy to play.
Before I got into the Halo series, this demo was what originally convinced me to want an Xbox at all, since Playstation 2 and GameCube ports hadn't been announced at the time. The actual game, however, suffered from strange, out-of-place objectives, such as having to push someone else's car along a very narrow underground bridge with no side rails, as well as an overall lack of polish in the level and mission design. Now I kind of wish I could have just played that demo over and over.