1. Understanding and hearing compression. Regarding any technical knowledge, this was easily the hardest thing for me to truly comprehend.
2. Realizing "analogue" is a horrible and widely misused word when describing a sound.
3. Learning to appreciate the relationship between the music and the mix. When I started out, I struggled to get the mix into what I felt like was a good mix and I completely forgot about the music. After being asked to jump to the second chorus and realizing I had no idea where and what the second chorus was, I completely re-thought my way of approaching mixing. This was a huge eye opener for me. There are thirteen AE's in a dozen who can get a decent mix going, but one in a thousand who can create something unique that really flows with the music, and that's something I strive to achieve some day, one step at a time.