1. Basic electricity
2. PWM
3. Battery chemistry and care
4. Transmissions/gear ratios
5. How DC motors work. Winds and turns and brushes, etc.
6. Suspension basics
7. Basic RF (crystals and channels and avoiding interference)
8. Soldering
And all sorts of hard-to-list mechanics and know-how.I didn't realize it at the time (and I don't think my parents did, either), but I learned so much more from that hobby than I did in school, for those areas.
3D printing feels the same way. I only recently got the Monoprice 120mm^2 unit, and it's working great for what I've thrown at it so far.
What are you using for all the non-plastic parts? Do you print slides and bracing as well?