That is truly a rhetorical question because there are so many types of Geeks that seemingly have nothing in common, ok maybe wire rimmed glasses.
I propose the following thing is the only thing required to be a Geek. A Geek must continually thirst for new knowledge. The knowledge does not have to be useful knowledge although most times it is. Learning is joyful not burdensome. I am sorry if you no longer live to learn turn in your Geek card when you exit.