Paul Nation, in one of his presentations at Kotesol mentioned that the best vocabulary activities do not require a lot of effort from the teacher. Does increased effort by the teacher result in increased learning or does it just assuage the nagging guilty feelings that you have about being a good teacher? After all, it's not the teacher who doesn't know the material, it's the students, so shouldn't they be the ones struggling away to learn it?
I'll extrapolate this modicum and apply it in a broad stroke to all teaching that is not content based. By this, I mean mostly conversational or speech kinds of classes (and writing to some degree as well). Of course, if you're teaching a Western culture class, then you'd need to do a significant amount of preparation.
So your classes? Ideally, the students would leave, having used their brains a significant amount. And hopefully, your voice would be barely strained because you'd only have talked for a few minutes out of every hour. And in theory, everything you do in class would be simple enough that you'd never have to take more than a couple minutes to explain it. If this is the case, I think that the students will have learned at least a little bit. Or, worked on their fluency, practicing using what they already knew.