BioWare wants people excited about Star Wars: The Old Republic, for example, and so the team wants to drum up enthusiasm for the best and brightest portions. But when release day comes, the countless ideas about what might be in a game collapse into a single view of what's actually included.
If you read news sites (such as Massively, for instance) on a regular basis, you usually get plenty of information on your game of choice. The question is, how does that match up with release? What did you think about your current game of choice prior to playing? Did you feel like you knew what was going on from all the pre-release information, or were you somewhat disappointed by the end results? Or did you think you'd never want to play the game based on previews, and then find yourself enjoying it once it was released?