Awesome amount of character building going on in this installment.
With Cox taking over the Chief Job, he actually has a friend in Kelso now, who devoid of the burden, is now actually turning into a great guy - the kind of guy whom we saw but mere glimpses of earlier (like at the end of s5ep04 - My Jiggly Ball - with Sideways by Citizen Cope playing). Advising Cox and JD both, this new turn for Kelso is the best character development I have seen in recent times.
The burden in turn has been shifted over to Cox, who is beginning to reel under the pressures of the title of "Chief". Not getting a single minute with the patients for a doctor who was there all the time for them must hurt, as well shown when JD says "your day is over"
For JD, the becoming the new "Cox" will be a tough thing, but it is needed to keep the place going, like Cox had done for ages with Kelso.
And good character development for Elliot too, getting along with others....
All in all, gearing up to a great season.