Djokovic had an exceptional season, won 10 titles including 3 GS and M500, gathered over 13,000 points.
That said, he looked extremely bad post-USO (6-4 w/l record), dealing with his own body and exhaustion.
It seems almost impossible to repeat such a tremendous season, personally I don't believe he'll be able to push himself to the limits once again and play so many matches with this kind of consistency.
I assume his own body could reject to bear such an effort in 2012.
He doesn't have to repeat it, it will be enough if he performs better than Nadal who also has a lot to defend.