I would even narrow that list down to one: Harmonics. Once it clicked in my mind that every sound is just a combination of sine waves, and that it's the intervals, amplitudes, and dynamics of those sines that make up everything we hear, it made sound design a lot clearer for me.

Of course, finding the right waveforms, filters, and envelopes required to get to a particular pattern of sines is still the challenge, but having that understanding of the medium underlying it all makes experimentation that much more productive (and fun).