Upload & Sell: Off
Size, weight, cost, different batteries, different GPS unit, lower resolution, no SD card slot, narrower bracketing range... there may be more, but I've only used a 1Dx once, and those are the things that immediately spring to mind.
The dual CF slots of the 1D-X are much more practical to me than the super-slow SD slot on the 5D III. Regardless of the card speed, the SD slot was too slow even for my landscapes. I'm hoping to actually use the two slots regularly in the 1D-X.