Fire is the element of power. The people of the Fire Nation have desire and will, and the energy and drive to achieve what they want.
Earth is the element of substance. The people of the Earth Kingdom are diverse and strong. They are persistent and enduring.
Air is the element of freedom. The Air Nomads detached themselves from worldy concerns and found peace and freedom. Also, they apparently had pretty good senses of humor!
Water is the element of change. The people of the Water Tribe are capable of adapting to many things. They have a deep sense of community and love that holds them together through anything.
It is important to draw wisdom from many different places. If we take it from only one place, it becomes rigid and stale. Understanding others, the other elements, and the other nations will help you become whole.