Good comment, yes, I've looked into this too. If you look at game design, incremental variable rewards are one of the most addictive possible things. Incremental means you unlock them gradually, and variable means you don't always get the same thing. That's why getting an awesome item in Diablo II was always more addictive than getting an awesome item in Baldur's Gate. In Diablo, you never knew what you were going to get and how valuable it'd be. In Baldur's Gate, the Hammer +3 was always with this guy, the dagger +2 was always buried down in the southeast tomb, etc.

Mixing variable and incremental rewards does increase addiction, which can be positive or negative. It's positive if you're harnessing it.

I think this could definitely work for teaching people things - one of the ways I learned how to type really fast was playing a text-based player vs. player game way back in the day. Getting semi-unrelated rewards (like you said next track, toughest unlocked, stump the teacher, or more commonly badges, ranks, points, etc) can be a massive boon to learning something. There's also the "head fake" element of teaching while not looking like you're teaching, which I got from Randy Pausch's Last Lecture (highly, highly recommended if you haven't seen it).

With all that said, I haven't found quite the right way to harness it in my own life. A lot of the tracking I do gives me a way to get numbers and feel good, but assigning myself badges or points seemed like a little more trouble than it's worth. Just setup and maintenance time of that systems seems like it wouldn't be worth it, but if someone else built a good one, I'd use it.

I've played with various software that tracks goals/habits/etc, but I keep coming back to minimalist, flexible stuff, that lets me do what I want with it. But I think sooner or later someone will build something that works well for either general education or other skills training, and I'd be all over that.