A good administrator understands that that they have to continue to learn and grow. They strive to do their best at every effort, admit to failures and learn from them.
They are extraordinarily communicative. Not only are they able to explain complex technical terms to a layperson, but they go out of their way to act as devil's advocate to their own idea to make sure they're giving the best options to their client.
The are active rather than reactive. They anticipate problems and demand, and resolve them, if possible, before they impact anything. When a critical issue is discovered, they quickly and calmly diagnose the problem fully before attempting to fix it.
The best administrators know and accept that there is more than one way to do things, that not every problem is a nail, and that because there is always a better way to do something than how they were planning on they constantly seek to learn new techniques, technologies, and understanding.
A good administrator loves servers, and has more than a few at home even if they already spend 80+ hours a week as a sysadmin.
A good administrator knows that she/he is not a great administrator. Truly great administrators do not exist, there are only those who claim to be great, and those who know they are not great.