there is one feature i really miss on any other camera and which only 1D series has:
exposure metering related to selected AF point.
also f/8 autofocus possibility is great too.
stop being oriented just about highISO performance, currently the noise level gaps between Canon cameras are narrower than ever even across the format if you shoot RAW. Other features are important as well.
From the 7DI to the 1DX/6D, there is a rough 2 1/3 stop gap at high ISOs in raw content. There are 6 cameras occupying this space currently, including the 7D2 due to be out in 2 months. The APS-Cs occupy the first 2/3 stop, the FF occupy the last 2/3 stop, with about a 1 stop gap between the APS-C and FF.
7D - 70D/7D2..... 1 stop ...... 5D3 - 6D - 1DX
The older 5D2 and 1D4 sit right in the middle of that 1 stop gap.
That is my assessment after having worked with raw files from just about all 8 cameras, either personally taking the raw shots myself, or supplied by others. Copy variations will shift one's assessment one way or another a bit, but on average, this is pretty close to what we see. A 1 stop advantage is not very narrow, because that is about the limit of how much noise one can post process out of a file while still leaving respectable amount of detail to work with.