i'm going to give kerr a little slack here...
he understands that the way this team was going (all offense and no defense) would NEVER win a title in phoenix.
so he brings in a new coach with a new philosophy and it's very PAINFUL for this team to make this change, notably steve nash who still makes remarks regarding this new direction.
kerr is put in a tough position, and he's going to end up blowing that team apart.
D'Antoni's Suns were average defensive teams.
This season Suns are a terrible defensive team.
I think that is more a player issue than coaching. Marion allowed them to do a lot of things on the defensive end, Diaw too. Plus it doesn't help that many of the other key players keep getting older, Bell was a very good defender the first couple of years with the Suns.
Another factor that does relate to coaching is the effort. The Suns believed in D'Antoni's system and that translated into effort on both ends. The upheaval has the team down. Its hard to fight for post position when you don't believe you have a future with the team.