What I consider the "Fundamentals" of audio and what you should master.
Reverb/Delay (Space and Width)
Harmonic Excitement/Saturation/Distortion (Filling the "gaps" of sounds and strengthening)
Gain Staging/Leveling (Getting a good blend where you know what mix elements get priority)
I've etched this deep into my mind and it is all I can really think about. I'm currently working on learning the different types of compressors such as FET, Vari-Mu, Optical (Opto), and VCA. Next level of learning will be focusing on more intermediate techniques such as Flanging, Chorusing, and Automation.
This is just some of how I am thinking, applying the 80/20 rule to audio mixing. I'll throw in some creative effects but mainly I'm going to focus on these fundamentals.