I like that you can set your own pace. Even then, I felt I wanted to push myself a bit out of my comfort zone because of the coach's encouragement. It felt great to break out of my routine and just go for it!
The small group environment but the feeling of a personal trainer who can help you with selecting weights or correcting your form. That they want to push you to your best but also know that you are not always at 100% and may need to just get through the workout and not get pushed.