Yes this is the big difference. And also just luck. It went his way last year in close matches; this year not so much, apart from the A.O.
I don't buy the "luck" factor for Djokovic in 2012 comparing to 2011 :
- in the only slam he won, luck was in his favour both against Nadal and Murray.
- in Roland-Garros defeating Tsonga was a miracle.
Only against Fed in the US Open did he have this kind of luck in 2011. So no I think the "luck" factor is overrated by some Djokovic-fans this year. Most importantly he was more fragile imo.
I can fully understand that Djokovic-fans are disappointed by his US Open and also Roland-Garros and Wimbledon losses, but he lost them much more because of his own failures than because of the external elements which are brought by some :
- in the US Open, there was some wind but Murray choked so much the first set that Djokovic should clearly have won it ; second set Djokovic had a disastrous start without which he would have won it ;
- in Wimbledon, he was not good against Fed, whether roof or not ;
- against Nadal on clay, he started too slowly and failed in fourth set, he was one break up when the match started second day and Nadal was very nervous ; besides, in Roma he had everything to beat Nadal once again if he had not made a huge number of sloppy errors, and surely that would have prevented Nadal from getting back confidence.
I mean I can understand it's very disappointing but he was not near from winning those matches, it's not like some disappointments having match point or being 2 points away from the match ; he was clearly too fragile.
Besides, the only slam he won he was much nearer to losing it than he was near to winning the others.