I have two requirements of martial arts, and although not exactly Yin and Yang, they often seem at odds with each other.
#1. They must have real and true combat applications.
#2. They must encourage and stress perfection of character and improvement of self.
On one hand, what you learn must be effective. It must be capable of causing injury and pain to another human being.
On the other, the very act of gaining this knowledge should reduce the chance of you ever having to use it.
"Si Vis Pacem Para Bellum"?