One interesting thing about the Avatar's vs. the Navi is that the Avatar's carry over some of the genetic traits of their gene source (human).

For instance the native Navi have 3 fingers and a thumb while all the Avatar Navi have the human 4 fingers and a thumb.

Look at the nose on Sigorni Weaver's Avatar. It maintains more of the human shape as compared to the other Avatar and the Navi lion like nose.