Without a doubt, the biggest gaming phenomenon of Japan this year involved the game which had anime girls as battleships. It's too bad that most people outside of Japan can't play it.
When I first wrote about Kantai Collection (艦隊これくしょん) – aka Kan Colle (艦これ) – it was back in July and the game was proudly boasting over 150,000 registered users. Pretty respectable numbers for a browser-based game. Of course, such celebration may have been a little premature. At the last official count, over 1,400,000 people (including yours truly) have registered to play. (For reference, in Japan, a browser game can be considered "successful" if it has over 20,000 users)