Its very difficult to beat Djokovic when he is not missing. Unlike Nadal who primarily relies on his footspeed or Murray who relies on his superb anticipation skills to retrieve balls, Djokovic plays a different type of defence and uses his flexibility to retrieve balls. His flexibility allows him to slide on HC making his lateral movement on the court very effective that even when he is in a defensive position he isn't pushed 10 m behind like Nadal thereby not allowing opponents enough angles so opponents have to either hit through him or come to net thereby making errors. One thing to note here is though he doesn't move as well on grass as its not easy to slide there.
Why can't everyone just hook the ball side to side like Murray did at USO and try to outgrind him? Well firstly there are a handful of guys who can match Nole's stamina. Not sure why you say its overrated? The man freaking won AO after playing 11 hours of tennis in 2 days (thats another thing though he was lucky or not during the match, thats subjective
) and has played the most tennis this year.
Secondly, Djokovic has an uncanny ability of keeping the balls deep even on the run converting defence into attack like none other so opponent who is basically trying to move him side to side is immediately put on the backfoot if he is not good at taking deep balls early. Thats why players who have a large swings like Berdych, Nadal struggle against Djokovic. Federer and Murray have far more compact swings so they deal with those deep balls well (Federer can even half volley from baseline actually
). And ofcourse both these guys have a good slice in their arsenal which others seem to lack and Djokovic doesn't like low balls that much.
Btw I dont think USO 2012 final is a good barometer to judge.