The Flash demo site is full of information(
http://web.canon.jp/Imaging/eos1dm3/), but its best feature is the Shooting/Custom Functions demo. A few very interesting developments show up which prove that Canon, indeed, was more interested in debugging the previous 1 series cameras than making a revolutionary statement. One example is the possibility to reverse the control wheel operation so that P, Av and M changes all work in the same direction. On the Mk2, it is still something that catches me every time.
Another welcome change is the mirror lock with Set option. If it is what I think it is, because they call it "Down with Set" when it should really be "Up with Set". Hum, maybe it's just the old one except you can leave the mirror up between shots and only bring it down when needed? Idiots.
There are built in limitators to what setttings you can scroll through such as use P and Tv but not Av and Bulb. Could be useful and works with ISO, speeds, apertures and metering modes. Spot meter can be mapped to AF point. Stop button on long lenses has more options, including switching from AIS to One Shot. The * button is now AF start with a longer list of options.
The quick control dial can be used to quickly change ISO while metering and ISO can be set to change automatically when set exposure becomes unusable. Cool.
The highlight tone priority is interesting but just seems to shift exposure to the left. If it did more than that, it could be very useful.
So it seems there are a lot of improved features but they beg to be tried for real world usefulness.