Let's see.....Djokovic only won one slam, and it was the closest slam final of the year, and it took him 6 hours.
This was despite 2012 being a very easy year, with Nadal missing for half the year, Murray not really arriving until the Olympics/US Open, and Federer winning Wimbledon but generally unconvincing at the slams.
2013 is going to be twice as hard to win slams. A well-rested Nadal (and he beat Djokovic at Roland Garros - and 2 other events - despite living on painkillers), a fully-established Murray, an improving Del Potro and the fading yet still there at Wimbledon Federer.
So Djokovic can rule out Roland Garros and Wimbledon. Nadal wins Roland Garros in his sleep, while Murray and Federer are the favorites at Wimbledon. They are not his. That leaves only the US Open and Australian Open. In early 2012 Djokovic had a huge edge over Nadal mentally. I expected Djokovic to win in 3 sets, given the roll he was on mentally at the time. Not the case, 7-5 fifth set (the same score as his semi vs Murray). US Open, clearly Murray prefers this surface and is only getting better, although as I noted the AO too allows Murray to hurt Djokovic these days.
Djokovic scraped through for a slam title in 2012. Don't expect 2013 to be as accommodating. And certainly in terms of motivation, Nadal and Murray will be more hungry than Djokovic, for obvious reasons. Djokovic is a mental player, relying on sources of motivation. Winning 'player of the year' in 2012 isn't exactly healthy motivation for a player who almost went slamless.
