To answer your question, the #2 (or thereabouts) problem I see with an intermediate caster is the wrong application of power. They almost have it down, but for the timing.
I think the key to a good instructor is the ability to not only diagnose the problem but to COMMUNICATE why it's happening and how to fix it.
Practice good. Bad practice bad. You're just putting nasty stuff in muscle memory. Bad habits are a true-to-life bear to break. It's best to learn propery from the get-go and practice GOOD form.