One of the results of teaching your own classes is that if you are doing your job to attract new students, it means that you are constantly teaching introductory material. This means spending a LOT of time on the basics.
A unique challenge I've been working on is balancing the needs of my junior and senior rank students. I think the top end of the class may often feel neglected by a class, and in fact most black belts must make some degree of personal sacrifice in class to work with junior rank. The reward is usually that their understanding of the basics increases and they can apply those same lessons to their current material.
For me, every time I teach something, it is brand new again. I get to look at the material I am presenting, tailor it to my crowd, appeal to different learning styles, ages, cultures and I walk away from the lesson with a better understanding than when I began. That is a great feeling!
All of this is great, but you must still find a time to cover more "advanced" material as appropriate for other ranks. While white belts are learning spinning back kick for example, green belts can be doing hook kick, brown belts can do wheel kick, red belts can do jumping variations of these techniques, etc. Or multiple kicks.
In my opinion, this is a very difficult challenge for most instructors: running several curriculum at once in your head and making sure everyone's needs are met.