Becoming member of a political party.

I met a lot of smart people, had many great debates about different issues, yet most of all: I learned how dangerous group dynamics can be. (It's insane how fast Us-vs-Them-group-thinking can manifest itself.) I learned to reflect myself (the hard way) and that if I want to convince someone, rational arguments is not enough if you are a dick about it and that sometimes the how you say things is so much more powerful than the what.

Basically, I learned a valuable lesson on how (not) to communicate. I still profit from that on a daily basis in my work as a developer.

(On the other hand, the whole experience made me rather cynical about the state of the world at large.)