I can't speak for December -- we were there March 1st this year and it was snowy and chilly (but warmer than Boston), but at least we had 10-11 hours of daylight. In Reykjavik, in December, you'll have 4 to 5 hours maximum of daylight. Not enough time to do the Golden Circle. In Akureyri you'll have 3-4 hours of daylight.

Everything I've read suggests South Iceland is pretty passable all year as it's warmer than the northern part of the country. Mountain passes are a bit scary, though. We went from Vik to Reykjavik during a storm and although we saw a few plows, there was a pretty consistent 3/4" icy mix on the road.

I love Iceland, but you won't see me there between November and February.