I'll add that workflow enhancements are nice. The 7D has a joystick, but the 70D has an 8-way dial that has the wheel and OK button built in. I found that I would fat-finger the button/wheel when using the dial and that wasn't fun.
When I initially upgraded to the 60D one of the features I wanted was to be able to reverse the direction of the wheel's effect on aperture (in Av) or shutter speed (in Tv). At the time it was a benefit to me that the wheel move the light meter reading in the same direction (as in, adjusting the wheel to the left moved the meter reading to the left)*.
Some other features that I want the 1DX for are better battery (I understand that it has a higher voltage than the others which results in faster AF), a 100% viewfinder (for composing, sometimes it's annoying when you get things in the frame with a <100% viewfinder, but it can always be cropped out), dual card slots (the 5DIII and 7DII both offer dual card slots, but it's CF/SD combo, not dual CF, which ends up being an either/or in most cases; the 5DIII slows the CF down to the SD when used together, no word on if the 7DII does that yet or not).
Some people like how a camera looks for various reasons. If you have an obviously pro body it can give customers some sense of confidence in your value (same reason you show up in nice clothes and act professionally). On the other hand, travel photographers, or photographers that work in public, but don't often work in front of their customer, may prefer something a little less obviously expensive.
Also, there are some more nuanced differences in how things work. For example, the 7D and 70D share the same autofocus sensor, but there are autofocus features (I believe this one in particular is called AF-expansion) that are disabled in the 70D.
One thing that bugged me about trying to find a compact(er) camera for travel is being able to do long exposures. The Canon P&S cameras don't have bulb mode and the EOS-M doesn't have a cable release port (it can use an IR remote, but that doesn't work for long exposures/bulb mode AFAIK). I ended up going with the Sony RX100mII.
Not all Canon cameras with a popup flash will trigger off camera flash units (I think most/all of the recent ones will, but I remember this being a problem in the past).
*Canon meters read -0+ and I understand that Nikon meters read +0-. So Nikon may do this right out of the box because the meter is backwards.