Well, at least if you listen to chatter here and elsewhere, it would be hailed if it can find focus in dim light without an embarassing pause, and a seizure-inducing spell of of pre-flashing and buzzing.
Actually, I would be happy if it could work well on the center point, including in dim light, and then work as well (or even a little better?) on ISO noise than the 5D3. Don't know what use I would have for GPS, but then I don't use video either, and yet it is in all the cameras.
Really, the ISO thing is my only real criterion other than the inherent properties of "full-framedness".