I've got to say, I'm a little shocked by this thread. I hate the "you have to watch the games" response, but that's the case in this instance.
One could point out that KG's stats were partially kept down by his limited number of minutes (32.8 mpg, by far his fewest since his rookie year). They could mention that teams only averaged an absurdly low 86.6 points per game when KG was on the floor as a PF, or that he held opposing PFs to an eFG% of 42.1%, well below the league average. KG controlled the paint, with opposing PFs only scoring 34% of their baskets inside or close to the hoop. He limited the PFs he covered to 10.7 rebounds per 48 minutes (while pulling down 13.3/48 himself), and he only averaged a personal foul against opposing PFs once every 12 minutes.
I could point all of those things out, but they don't really matter, because the best demonstration of KG's defensive impact comes from observing him closely on the floor. In doing so, you understand that he clearly deserved the DPOY, from his hollering out instructions to his teammates, to cutting off drives (allowing Pierce, Rondo, and Ray to be more effective on the perimeter), to boxing out on rebounds and keeping his man out of the paint. Stats be [dang]ed; KG was the best and most influential defensive player in the league this year.