What I learned from participating in my second (third) Game Jam and what came out of it (Devils Tower).
Even back when there was no fancy 3D real time rendering, I wanted to mess around in virtual worlds and experience, what I couldn't in real life. This inevitably lead to me wanting to create computer games myself...
Constructive Solid Geometry, using Boolean operations to create more complex geometry out of simple, manifold shapes.
A toolbox for procedural geometry generation.