I just read the Q & A and we actually do already have an answer to the change in direction.

What made you decide to focus more on character development this season ?#SherlockQA

For a third season it's important not to rest on your laurels and we wanted to explore different approaches ^MG

That approach did completely feel like the were resting on their laurels, though. To me season 3 doesn't look like they put as much effort in it as previous seasons. I know it's just my point of view that makes me think this, but I the case side of this show is rather important to me.