Being with a group. Everyone is doing their best, making the most of the workout! I think the stretching at the end of the workouts have helped me immensely, and prevented overuse injuries, which are common in these type of workouts.
Structure, variety, challenge, desk staff, trainers.
I wish trainers would correct those with poor form like they used too.
Also, figure out a way to keep late arrivers from crowding the door to get treadmills.