Not bad for your first go... If you dont want the feet to slip just used clamped keys.... As far as the knee's... i'd have to see how you rigged him with Ik's, to get the root to move down with the feet staying put.
I am enough of an artist to draw freely upon my imagination, knowledge is limited, imagination encircles the world. (Albert Einstein)
https://www.artstation.com/kurtb