Like datajunkie said, there is no coasting or what not. The no coasting is hard enough.... but the what not.... that's the killer.
You've got to really hydrate. There is little air circulation in these classes, and you've got to water up. There is also a propensity to think it is so easy that you spin yourself tired earlier and don't pace your hour.
And lastly, one of my biggest pet peeves.... there is inadequate warm up. Many times the instructors have you up and out of the saddle too early and ramp up your heart rate much too early. You've got to take charge and make sure you are warmed.