Support for the EC-S focusing screen. Seeing the dof and focus is a great aid and truly missed from the 5d2.
i discovered that the 1dx takes all the old 1dsmk3 screens- all the EC focusing screens. i used the ec-s in a 1dx and it was soooo nice- i shoot at f1.2 a lot, so this made a massive difference.
- things i would like improved-
1/ magnify button moved back to where it was on the ads.
2/ the lighting up of the focus points made at least twice as bright, this projecting on the focus screen cannot be seen when shooting a lot of subject matter.
3/ clean hdmi out for video
4/ 60fps at 1080p (or 120!)
a more dreaming list-
A/fast wireless tethering
B/a way to shoot that you can have live focus through an external monitor and shoot stills without any delay (its too slow at the moment), better yet, allow one monitor to frame the shot, and allow a second monitor to have the image zoomed 100 percent (great if you can dedicate a focusing person on those real hard focus problems).
C/more focus points spread out wider around the frame. i did a quick look at a whole pile of advertising photos and the focus points where they are now do line up with 60% of the shots i saw. most were outside the focus points in the middle.