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.