"difficulty builds mental muscle, while ease often builds only confidence"*
I teach an engaging interactive class. Students love being in the environment and report learning a great deal. When I can keep class size to 15, students report that they have to work hard to keep up with the discussion, they can't hide. Bigger than that, and I know there are some folks who, while they are listening and learning, they're not pushing themselves hard to build those mental muscles.
Those who do push themselves build those mental muscles. It's clear in their work products and in-class comments.
Yet, other instructors and often students assume that because they're having fun, the class must be easy. Students seem to feel betrayed when they earn a B or C -- their work floats above the surface, they're not digging deep, and they've earned the grade they've earned. And administrators and other instructors overlook the grade distribution noticing only that students like the class so it must be easy.
What a weird position to be in: I can get people working their brains hard and reward those who dig in and do well, but because they work their brains hard in a way that's engaging, everyone assumes the class is "light". If I stood there and talked at them -- which I think would be a far easier class as I spoon feed them ideas -- everyone would think the class is challenging.
*From: Come On, I Thought I Knew That! By Benedict Carey (http://www.nytimes.com/2011/04/19/health/19mind.html?pagewanted=1)