Players are often worn out by year end. Nadal has complained of being tired & injured for YEC before. Roger wasn't feeling 100% in 2008. Del Po's wrist was probably a factor in 2009. It's actually not a unique situation this year.
There's not much wrong with Murray. Probably not with Nadal either. Djokovic is not 100% but his injury won't prevent him from competing. Federer is in good form and he enjoys the indoors. Berdych is picking up some form despite of his defeat today. Tsonga is dangerous too. Ferrer is Ferrer as always, no big change here. Fish is half-injured sadly.
Personally I think it will be quite interesting to see how it pans out. I hope that the big 4 will get challenged this time.
We should be appreciative of Federer though, because we will never see anything like him again. The fact he can still compete for slams ever after the erosion of his physical skills is really a greater testament to the natural talent he has than the period of time when he was dominating the tour.