The opposite is true in the summer. Humid days create a stark contrast. This weather is great for the whitewater kayaking I like to do in Japan, since I live between two canoeing rivers just 1 hour from Tokyo. Japan is of course a surprisingly long country, though tilted on a NE-SW axis. I would however suggest that the NE (i.e. Hokkaido) is far colder, and the SE far warmer. This is both because of latitude considerations and the influence of ocean currents.
For more climate details for Japan, check out Wikipedia. The great news is that Japan is appealing at any time of year. There are great ski packages. You can be based in Tokyo, and jump on a shinkansen (i.e. Very Fast Train) to say Nozawa, just on the other side of the range, in Niigata prefecture. In summer, the same region offers canyoning, MTB and rafting activities.
Most people go to Toyko, however I am very fond of some of the larger cities like Hiroshima, Fukuoka and Nagoya. I do however suggest getting a Japan Rail Pass which offers a huge concession for holiday-makers, and travelling to some of the smaller, more remote areas. I particular was keen to take all the train routes that followed the river valleys. They are often slow trains stopping at all stops. I have a collection of GPS coordinates for all the train stations I have been to. Thought it might come in handy for future canoeing/kayaking. i.e. So I know when to get out of a river and walk to the station with my inflatable canoe. The landscapes are pretty special. The culture is not simply in the cities.
There are some pretty unique people around. Some worthy of mention:
1. Small bar in Matsue offers some of your obscure alternative rock bands from home, well in my case Australia & NZ. Anyone for Straightjacket Fits, The Chills or Ned's Atomic Dustbin. I didn't get everything I wanted. The guy has a collection of over 6,000 CDs.
Sorry, its a short list, as all of my other experiences were in bars, and I can't remember them, but you get the idea.