What exactly are Djokovic's strengths? Is it his speed? Defense? I've never been able to tell.
He's great in every part of the game even if he doesn't really have key stroke (on the opposite, he doesn't really have weaknesses).
He's top 2 returner of this era maybe OAT (can't decide if it's him or Nadal (or Agassi))
Top 2 defender of this era (between him and Nadal)
Top 5 2h-backhand of this era
Serve, Forehand, Footwork, Stamina are top notch.
Before his invictus year, he was already one of the few (with Federer basically) who was able to really compete with prime Nadal on clay producing classic 3 setters ('09 Madrid, '08 Hamburg)
Plus, mentally, since he won the Davis Cup for Serbia, he's on another level of confidence.
He also may be a little insane :D because he tries silly or highly risked things in unfavorable situations (against Fed in the US, Tsonga in the French for example) that makes him absolutely dreaded.