It's worth saying that technically they have hugged before, but that was initiated by John at the wedding. This is the first time they've hugged and it's been initiated by Sherlock, and that's why I'm emotionally compromsed.
Eleven--they had Matt smith dress up as Sherlock Holmes because he read for the part of Watson in Sherlock and they thought he would make a better Sherlock Holmes. My theory. It was probably their nod to that.