Sorry to break to you but Novak is a great player but not an outstanding talented one to the likes of Federer or Nadal level or any other outstanding multi slam champion for that matter. I would even put Nalbandian as slightly more talented and he hasn`t won a GS slam yet.
Djokovic is a great top player, he has a very well rounded consistent game, great FH a solid and steady backhand a nice serve and descent volley skills and mental. Having said that he doesn`t possess anything outstanding in his arsenal neither technically or mentally. When he gets hot mentally he can have great results, I remeber I think just last year where he was one of the players that most pushed Nadal on clay specially that endless final in Hamburg I belive, which could have lead to Nadal eventual tireness, physycal issues and loss to Soderling. Slam finals, I am not sure how many MS 1000 he has one but I think a few...etc..
But in the end is just that. Djokovic is a great top player, but he will not beat a Federer playing really well on any surface nor Nadal for that matter maybe aside from fast HC. He is also the type of player that can lose to several other good top players on any given day. So is just that, he is quite similar to Andy Murray, having achivied more than him yet as far as GS and on clay as well. I think as a Nole fan you can`t complain too much, I think he could problably achievied maybe another slam and a few MS 100, but either way he is career is going just fine in accordance to his game. Specially when you consider that guys like Nalbandian, Haas, Murray that has won as big yet....
I also think he is similar to Delpo in the sense that the Argentine won it big and young, but he doesn`t really have that talent or expectations of winnning GS left and right. And I think most people would not be surprised if he won just that US open, for example. Given that he is great top player,but not also an outstanding player technically or mentally.
Awesome write-up, but I think you're wrong about his talent level. As for Nalbandian being ahead of him in that department, I'll still give you that. Not only is Nalbandian naturally talented, but he has a really great tactical mind which is a whole different type of talent in and of itself. But when Nole was really at his best (in the days of the Big Three/Four), his elasticity and flexibility made him one of the best shot-makers on tour. Federer, Nadal, and Nole had probably the most consistent passing shots on tour, and that's no coincidence because they are all flexible and athletic enough to hit any shot from any position. Even looking at the tour now, and how much the game has evolved, there are very few players (maybe Monfils) with Nole's athletic talents.
No, I think where Djokovic went wrong was entirely mental. As others have said, when he lost that youthful cockiness, that fearless shotmaking, and began to mature, his game has suffered for it.