I think there are two kinds of D students - and I have an inside scoop on this one. There are D students who simply can't earn a higher grade due to intelligence or life factors (such as moving a lot and missing a great deal of background information) and then there are the D students who are simply unmotivated in certain areas, but highly capable.
I was a D student only one time in the second part of Engineering Calculus in college, but that's a different story. I've always been an A/B student because I played the game well enough to get by. I did all the right things like testing out of 18 hours of college, doing a "powerful" business degree, etc... It was easy - in fact, even the Masters was so easy, I'm thinking of a Doctorate. I like learning, and the higher you go in education the more choice you have in what you want to study.
But despite all that, I left two very well-paid careers to become a teacher for altruistic reasons, so I don't fit the mold on this one. But I can say I'm the only teacher in my building who is at risk of being one of Obama's "rich" American households making $250,000 or more per year.