That demo just made me feel so hyped, from how Booker's and Elizabeth's actions seemed to be based on the player's actions (deciding to euthanize the horse, pointing the gun at a heckling Vox Populi guy, etc.) to the much more interactive enemies and environments (e.g., a bunch of enemies riding on the sky rails, using powers like Bucking Bronco in concert with Elizabeth's powers to materialize a cart and hit the floating enemies, etc.).
If the finished product had stayed closer to that, we probably wouldn't just be debating which Bioshock was the best, it would also be a lock for game of the year (which I don't think it will be, if some of the other games releasing this year turn out as good as they seem).