Safe & Effective Movement
Otherwise known as bracing the abdominal wall to support and protect the spine. If I had to pick only one thing to teach clients it would be how to do this properly. It applies to all movements both in and outside of the gym. Not only does this benefit the quality of training but also improves awareness of day-to-day posture and "core" strength.
Joint Position Based on Individual Anthropometry
Everyone has a different skeletal structure and bio-mechanical advantage. I've taught hundreds of athletes and general population clients of all shapes, sizes, and injury histories how to perform complex, compound movements. I pay very close attention to individual differences and how to optimize form to ensure longevity.
I see the "value" of a movement on a spectrum. The more muscles involved, the greater the range of motion, the easier it is to safely control and progress the movement, the more valuable it is to me. Not everyone can do a barbell low bar squat, but anyone who can sit in a chair can do an assisted high-box squat and can therefore make progress towards doing more demanding movements.