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 worldly concerns, and they found peace and freedom. And they apparently had great senses of humor.
Water is the element of change. The people of the Water Tribes are capable of adapting to many things. They have a sense of community and love that holds them together through anything.
It is important to draw wisdom from different places. If you take it from only one place it become rigid and stale. Understanding others, the other elements, the other nations, will help you become whole. It is the combination of the four elements in one person that makes the Avatar so powerful. But it can make you more powerful too.