Berlin during New Years is cold, fun, and just festive enough. Many of the Christmas markets last until after the new year, so the city still has the holiday feel. In fact, the market at Weihnachtsmarkt an der Gedaechtniskirche is one of the most festive Christmas markets we've seen and they shoot off fireworks every night. People are out shopping and eating and the city feels alive. This was our last chance of the holiday season to drink gluhwein, so we filled our cups every chance we got. Besides, it's just hard to resist the sweet, spicy aroma. On New Year's Eve the fireworks at Brandenburg Tor are first class and attended by throngs of locals and tourists braving the cold to ring in the new year. And yes, there was a group of douche-bag American guys near us that sprayed everyone with champagne.
We started New Years by eating Chinese food at Peking Ente Berlin in Mitte. While not entirely authentic [the hot and sour soup was a bit too peppery and thickened], the food was surprisingly good and moderately spicy [there is nothing spicy about German food]. The service was speedy, but then it was New Years Eve and we think the staff wanted everyone out early.
Berlin in winter has a different feel than Berlin in the summer. The cold and occasional overcast lend a certain gravitas to the city. It reminds me a bit more of Berlin 20 - 30 years ago. Still, winter is a great time to visit. The monuments and streets are not as crowded.
Berlin is a big yet surprisingly manageable city that is always changing - new hotels, new stores, new monuments, new things to do. They remodeled the food court at KaDeWe and thankfully the raclette bar is still there, albeit ascetically "upgraded". It's the one thing we do [raclette mit pickles] every time we visit Berlin. We also always stay at the Grand Hyatt in Potsdamer Platz. Somethings should never change.