The instructors- the classes wouldn't be so good without their enthusiasm, great loud music and constant consistent instruction.
Also I love the one hour in - out and you finished a great workout. No messing around or wasted time. Everyone - including the front desk staff are so nice and friendly it makes it enjoyable to show up.
All of it -- and I am out of shape. Mind you, I am sore as all get out on day 2 and I did throw up after, but all was worth it. I will be happy if I am addicted to these classes. I would like to see if they could offer beginner classes more, it is a little crazy when you start.