As a mediumly partially experienced turning hobby-izer (Done a few things dozens of times - but not hundreds - and there are a whole lot of things I have never started trying), the best things I have learned have been on proportion. John Keeton is the one who gave me a lot of personal advice by the Creek and by E-mail.
I would also like thorough instruction on one tool. Maybe the skew or the bowl gouge, spending time and showing grinds and positions and techniques and practicing them.
A third area is in more complex forms, how the parts are assembled together, how to move from paper to form with many parts.
Veni Vidi Vendi Vente! I came, I saw, I bought a large coffee!