-When you've got someone outnumbered and they are turtling up, you can make an opening
-When you're near an edge you can knock someone off
-When someone's in your grill you can push them back
-If a teammate archer/xbowman shoots you in the back while you're fighting you can go rough them up without leaving bruises
-You can launch a teammate (preferably wielding a great maul) into the enemy team, effectively closing the distance and throwing people off
-You can head bang
EDIT: oh, I guess you asked about kicks as well. Both kicks and nudged are situational, but in general kicks are good when:
-You are backpedaling and can predict where someone's going to be at the exact moment that they are in kick range
-Someone is trapped in some sort of way and you want to get a hit in (E.G. in a corner, surrounded by people, stuck in a glitch)
Really, just watch good players and observe when they kick. After a bit it becomes instinctive. Good luck!