Disclaimer first: in general it's really early to rate, but those are my initial thoughts and evaluations(take with a grain of salt):
Offensive system and execution: B
I think we can all agree our offense looks better than last year's. There is ball movement and offensive execution seems to be good, with some exceptions. There are moments when our system looks really smooth(for example the majority of the first quarter today). I think he needs to work on the pace and our transition offense... I also think they need to work on overcoming adversity and pressure. Right now our offense seem to break down whenever anybody tries to play really physical against us.
Defense: F
Horrible! Truly horrible! Need I say more? Especially after this was the main thing he was harping about and the main thing he said he was going to fix. We all know that the pieces are not the best for the purposes of the defense, but come on. There are so many defensive breakdowns, so many lapses in concentration. Some blame should be put on the coaches...
In-game decisions: C+
I don't think he uses the pieces we have the best possible way when it comes to match-ups in separate parts of the games. He'd let Griffin abuse Kanter any way he wants while one of the most limited offensive players in the league(Jordan) is assigned to the much better defender(Favors). He would leave Burke to be abused by much faster/taller/powerful PGs for way too long. He wouldn't demand Exum to exploit much shorter and weaker defenders(Telfair), etc. Some might think some of those are minor things, but still... it feels like there's room for improvement. He'd also sub out any player in the second quarter if they get a second foul... that's the thing that drives me insane the most. He's had some nice moments and nice adjustments. I feel like I need to credit him for some of the comebacks we've managed to make since some of them come either after screaming from him or after the half-time when he's had time to talk to his players and supposedly motivate them and show them things to exploit in the opponents or things to fix in their own game.
Development: too early to tell...
In general I'd give him about C for overall performance.