This week's update is the first of 2013, and brings with it a lot of updates, the most important of which are the new hero, Medusa, as well as the new game type, All Mid.
For a look at Medusa's video, check out her official page: http://www.dota2.com/hero/Medusa
All Mid is exactly what it sound like. The sidelanes are disabled, everyone goes mid, and you can have the same heroes on each team. It's a typhoon of fun. A funsoon, if you will.