Great question! I just learned something too! Turns out, the rest is mostly desert (such as Gobi), snow-capped mountains and glaciers. China's largest desert, Takla Makan Desert, is in this region. Some areas get REALLY hot. In the hottest month of July, the temperature in Turpan of the South can reach as highas 48.9 degrees Celsius (that's over 120F!!); whereas in the coldest month, the temperature is as low as ?51.5 degrees Celsius in Fuyun of North Xinjiang.
Only 4.3% of Xinjiang's land area is fit for human habitation because of the Tarim and Dzungarian Basin which are usually flooded. Another reason is the Mountain Ranges located in the South of Xinjiang. For more information, check out http://en.wikipedia.org/wiki/Xinjiang.