* The control joy/mastery feeling you get from activities like car racing, riding a bike on technical terrain, playing a game, or flying a plane[1].
* A sufficient risk/reward profile; you risk smashing a $100 device, which is expensive enough to be interesting, but low enough not to be paralyzing
* The gearhead thrill of ordering, rebuilding, designing, and discovering new combinations of equipment
In my case, I want to optimize for quietness and unobtrusiveness, which means aiming for lightness, which is its own set of fun design problems - I'm currently down to 25 grams for a brushless FPV setup[2]. Now I'm on to redesigning the RC transmitter[3], since I want one with nice gimbals that takes up as little space in my bag as possible. The set of things you can do is endless.
[1] I hooked up an RC controller and went FPV with my home flightsim setup. It was fun, but there's a reason RC sticks are tiny - you need to move them around a lot more frequently and quickly than in a plane: https://twitter.com/gmurphy/status/832698732361551872