The problem could be when you start the game sitting down, because the camera might have problems recognizing you height
Not only that, most people have coffee table between the TV and couch. e.g., Kinect may have problems seeing the foot movement for acceleration/brake in these cases.
It's a much more complex tracking exercise in practice. Demoing on the show floor may not be indicative of the real user environment (e.g., couch, cushions and sitting/leaning bodies are "fused" together, plus stuff on the table).