IMHO, a great DM is aware of their own strengths and limitations, and understands how to use their strengths to overcome their limitations. For example, one of the best DM I played under was fantastic at characterizations. He had an amazing command of his voice, his facial expressions, and his body. Moreover, he was capable of switching between characters at a moment's notice. On the flip side, he had a very poor understanding of combat tactics and wrote otherwise dull tactical encounters. Now, many DMs would see this weakness and look at ways to improve their skill. Maybe utilizing published encounters or honing their tactical acumen. Andy didn't. Instead, he accepted this weakness and realized that the time and energy spent improving his tactical skills would be better spent creating interesting personas to interact with. We never just had a combat with 3 by-the-book goblins; we had an encounter with Snarg, Twig, and Grengle - each with their own unique personalities. That made for an entertaining encounter.