It was hardly on the level of the J-20's appearance two years ago, but the advent of the Xian J-20 transport over the Christmas holidays was nonetheless important. If nothing else, it's the third all-new Chinese military aircraft to emerge in two years, a pace of innovation unknown since the Cold War. It is also by far the largest indigenously developed Chinese aircraft.
A lot of people are pointing out that the Y-20 looks a lot like most other military jet cargo aircraft, as indeed it does, because few people so far have successfully diverted from the formula that Lockheed-Georgia used with the C-141.
The aircraft is roughly the size of the Il-76 and uses the same engines for now (Saturn D-30KPs, also imported for the H-6K bomber). It is widely predicted that the production version will have a Chinese-produced high-bypass-ratio engine. Other significant details are yet to be revealed, including the design of the landing gear and the high-lift system, which determine the aircraft's ability to use short and soft runways.
Some see the Y-20 as the start of a family of special-purpose variants, including an all-domestic airborne early warning and control aircraft, but a large military transport - relatively heavy and draggy - is not really an optimized platform for AEW. The Soviets used the Il-76 because it was the best they had.
So what is the strategic mission for the Y-20? The US developed large airlifters primarily for the reinforcement of Europe, secondly for long-range strategic interventions. Russia developed them (along with a family of air-droppable vehicles) because of a strong belief in the power of airborne combined-armed forces. Some nations, more recently, have acquired them for a mix of missions, ranging from armed intervention to operations other than war - non-combatant evacuations and humanitarian/disaster relief. Exactly what mission mix the PLA has in mind is yet to be revealed.