Since the 6D is post-600EX-RT, I assume there would be some advantage to having a 6D over a 5DII for HSS off-camera, though I know some people are not feeling limited by the 5DII in that regard.
While Canon says otherwise, it has been confirmed by multiple users that hss & x-sync works just fine with 600rt + 5d2. You won't get the in-camera menus and the new option flash groups, though.
The real advantage would have been if the 6d had a rf-controller next to gps/wifi - would certainly have been possible since it's not full metal.
People are already pissed they put in the wifi and GPS since they didn't put it in the other bodies. If they'd done that then it would of been even worse complaining. Also, I think I read the range on the wifi is only 50m vs 150m external. So it might not provide as much range as an external option.
Actually, I thought I read it was more like 30m - but that's typical of small consumer electronics. You'll be hard pressed to do a real-world test with Bluetooth or WiFi outside of 30 meters (about 100 feet). Keep in mind that they only post what can be solidly achieved with no hope of failure in these tests. Usually when a manufacturer announces these kinds of range specs, most users can experience even double the performance. It will all come down to the particular scenario, i.e. indoors, outdoors, signal interference, strength of broadcast, movement of user, other environmental factors...
Also there's a hands-on example of the AF system available on the Fred Miranda forums that I'm going to post on a separate thread. It looks like the center point of the 6D is next to revolutionary, and the outer points outperform the 5D2 with ease (also, they're not assist points like the 5D2).