While it is still extremely early to speculate on the fate of AirAsia 8501, analysis of the ADSB position reports provided by FlightRadar24 reveals a startling fact.

The ADSB squawks report a steady airspeed, while the distance measured between each coordinate shrinks. By comparing the timestamp of each position report, and measuring the distance between each report, we can calculate an average ground speed. Plotting this speed along with the speed reported by the ADSB squawk itself, presumably from the ARINC 429 data bus, reveals this:

As you can see, the lines are well correlated until the final squawk, where the reported airspeed is 469 kts and the average ground track speed is calculated at 264 kts.

This points strongly to Pitot tube icing, and the aircraft stalling as it attempted to climb.

I have uploaded the raw data here: Google Drive

You can view the track points as flight plan here: skyvector.com