Stress
It'll be grossly hot in the suit and freezing cold outside the mall's sweet embrace. You'll be dealing with crowds of people who've been standing in line for hours with literally hundreds of impatient children. So, there will be plenty of tension to deal with. There's also the plain fact that some kids are terrified of Santa, no matter how rosy your cheeks or cherry-like your nose. Be prepared for some B-movie-level screaming every now and again.
There's also the stress of actually being Santa Claus, because that's exactly what you are to these children. That means they may ask you for things you could never possibly deliver on, like a pony or a new house or new parents. You'll have to empathize and bring joy, without giving them too much of a let down on Christmas morning when there's no pony, it's the same house, and mom and dad are still there.
Um...merry Christmas?