I've had a play with the 6D and it seems reasonable. Just playing with it in the shop, I did manage to get a 24-105 lens to hunt for focus using the centre point when I aimed it at a black poorly lit cupboard door. However, I doubt that I will be taking many shots of black poorly lit cupboard doors in my life.
I also realise that lenses make a big difference to the autofocus. On my current 550D for example, the 100L macro focuses in an instant whereas the 50 f/1.4 is relatively slothful.
I've been humming and hawing over the 5D III and the 6D for a while and I will probably go for the 6D just for the wifi capabilities. Sometimes I don't have time to faff about with RAW images and it would be nice just to be able to send some jpegs direct to my tab.
What has been putting me off the 6D is the autofocus. I was not expecting the 5D III autofocus system but I do think that cross type points should be standard - like on the latest rebel body. 11 cross type autofocus points would have been perfect for this camera and would have quietened down the nay-sayers. And yes in some situations the 6D autofocus system is just fine but in others it is wanting. I have small children. They rarely sit still. My 550D rarely copes with them running towards me when I think I'm sneakily photographing them. My Samsung smart phone compares favourably with my 550D in terms of nailing focus. Think about that. Granted, image quality is nowhere near but the best camera is the one you have with you and frankly carrying around a 5D III just on the off chance of getting a snap of my son on a swing in the park is not going to happen.
For me as a non-pro, I want one body. I also would like full frame so I won't be getting the 7D.
For us lowly amateurs, a camera needs to be a bit of a jack of all trades because we just can't be spending thousands on gear when it doesn't get used enough to justify having it (read: the missus would kill me).
Therefore, it is my humble opinion that Canon could have tweaked the autofocus system for the 6D to be just a little bit better.
Let's not forget that the 6D is an expensive camera. At £1500 in the UK, that to most people is a very expensive camera.
There's a reason why the smart phone copes 'reasonably well' in your situation. The tiny format has massive depth of field.
I know you were joking about focusing on a black door, but try manually focusing on something with no definition; it's all but impossible. A cameras 'range finder' style AF is no different, whether it be x type or not.
I agree this you that the 6D is, in its own right, an expensive camera.
It's also a damn fine one. And there's no 'owners pride' with me. I don't actually buy or own the gear I use.