The thing that blew my mind was how you can replace discrete transistor logic with memory. While it is sort of obvious in retrospect. I think it an important insight into the nature of computation.

Making combinational logic gates from transistors: https://www.youtube.com/watch?v=sTu3LwpF6XI

Using an EEPROM to replace combinational logic: https://www.youtube.com/watch?v=BA12Z7gQ4P0