Anybody who’s taught a history class can probably sympathize with the points raised here and here. In some ways, it’s harder to teach the material you know really well than it is to teach material outside your immediate area of expertise.
As John Fea says, when I’m teaching the stuff I’m really into, “I always leave the lecture hall frustrated. As I walk back to my office I often obsess about everything I did not have time to cover.” When you’re passionate about a particular topic, you want to give it the coverage you know it deserves. Of course, this is usually impossible, especially with a survey course. As a result, you leave the classroom feeling disappointed with yourself, and then you start wondering about whether you’re teaching any of the material adequately.
The lectures I’m most content with are the ones where my understanding of the subject falls into a sort of middle zone, where I’m familiar enough with the material to be comfortable but not so thoroughly schooled in it that I’m conscious of how much I’m leaving out.