1. Understanding exactly what the cutoff filter did. This opened up a whole new world in regards to opening up the cutoff for wobbles and more importantly - using envelopes to create my beloved "whomp" sound
2. Waveforms and how they affect sound. More specifically, realizing that the "sharper" the end of a wave form, the more abrasive it was going to sound. Ie. Sine waves for more "hollow" sounds, followed by triangle, square/pulse wave, then saw for really abrasive sounds.
3. Learning scales. I cannot stress how important this was to me in order to make anything sound decent. Spending a few hours on this (and I go back to it every so often) has really improved the musicality of my tunes.
Over to you, community!