The coaches pushing you and guiding you throughout the entire hour. The variation in the workouts! I never came in and did the same thing. I love the mixture of using the treadmill, rower, and floor exercises.
My favorite part is that your heart rate zone is visible on the screens throughout the whole workout. If I'm in the blue or green zone I know I need to step up the intensity... or if I'm struggling and look to the monitor and see that I'm in the red zone - I know that I can slow down, without feeling guilty.