very few campers around so you can usually get a good site and not be surrounded by lots of people (exception may be school holidays in popular camps)
almost no insects around to pester you
snakes are rarely a problem when hiking (but can be seen especially on sunny mornings when they try to warm up)
less traffic
you can usually have a nice camp fire in most camp grounds and you can cook up some nice Dutch oven dishes
ideal time to have that wood stove in your hot tent
great time for landscape photography as:
the winter sun is lower creating more aesthetic lighting, especially if you get some fog
the clouds tend to be more dramatic
there may still be some mushrooms around
there is usually more water running in waterfalls
great time for star gazing or Milky Way photography as the Winter Milky Way is the bet by far
it may be a good time to go looking for gold, if you have the gear as many of these sites are north of the ranges