I wonder if Novak is suffering a massive letdown after 2011. Some people win ONE Slam and then go through a pretty miserable period before they learn to deal with it (think Li Na, Stan and even Federer mentioned something like that after winning his first Wimbledon, I believe.) Novak won THREE, so it might just be a humongous case of mental and emotional indigestion.
I have no doubts that Djokovic still has another 4 or 5 Slams left in him once he gets over his current troubles and learns to balance his new attacking game with his natural defensive one. Let's face it, he is not a natural volleyer or net rusher (not necessarily the same thing) nor has he been the aggressive baseliner until recently only, so I believe it's only a matter of time and getting used to working outside his comfort zone, before he starts winning again.
If anything, the way he lost the FO this year must have reinforced the lesson that he needs to be able to end points quicker, especially when he considers his age and the toll playing a grinding, retrieving style is taking on his body.