Dwayne Casey was the defensive coordinator for the 2011 Dallas Mavericks, but that does not mean Casey is a great defensive coach. It means Tyson Chandler and Shawn Marion are great defenders. Casey's defensive schemes only worked because he had the personnel to execute them.
A good defensive coach without good defensive personnel is a guy re-arranging the deck chairs on the Titanic. A coach is only as good as his players. There are certain defensive schemes that NBA teams need to use, but there's no one being outsmarted tactically out there - it's just not a very complicated sport.
In general, the media focuses way too much on tactics and not enough on strategy. It's not about how Team X likes to play pick-and-rolls - it's about the personnel Team X has and the types of pick-and-roll strategies they are capable of using. In the playoffs, you have to be able to adjust that stuff anyway.
If you have fast big men and long guards, you can switch, trap or hedge and recover. If your big men don't move their feet fast enough, a coach is pretty constrained with what he can do. There are certain basic things a coach needs to do, but most NBA games are ultimately won or lost by the decisions a GM makes.
Coaching is a trade. You learn the skills of the trade coming up through the ranks. Any NBA assistant with sufficient experience could be the head coach of a team. There are no secrets to what's going on there - for the most part, it's about who has the best players.
The reason coaches are big deal at the college level is because they function as GM's too. A college coach's roster is a reflection of their overall philosophy, in terms of the types of players they are bringing in. An NBA coach is a caretaker with a certain set of skills honed over a long period of time.
The biggest effect a coach can have is in how they deploy personnel. Jeff Hornacek isn't making guys in Phoenix better because of his magic coaching beans, it's because he is letting them play in more space by utilizing stretch big men - Channing Frye and the Morris Twins.
The Suns players are this good in the system they are using. In a different system, like the two-post half-court one in Indiana, guys like Miles Plumlee and Gerald Green aren't as valuable. It's not about the coach who installs the system - it's about the players he has that can make his system work.
Mike D'Antoni is the same coach he was in Phoenix, if not better since he has so much more experience. The difference is he doesn't have Shawn Marion, Amare Stoudemire and Steve Nash in their prime. The most important thing a coach can do is pick his players.