I've noticed several people mention the 6D having 2 card slots. I think this is DEFINITELY a feature that will be missing. It's an easy way to differentiate between it and the Mark iii plus if it's really trying to get the entry level ff market then most people upgrading will only be used to one slot while those that are using it for a 2nd body will realize that's the cost of saving some money or if it's really important to them then they'll go get a 5d mark iii.
I think you could well be right and that the D600 and the 6D end up actually going after very different markets. The D600 to me looks much more a response to the 5D3 than it does an entry level model while Canon obviously have no need to come up with another camera in that area.
Could turn out to be smart business by Canon I'd say since I think price comes a much larger issue for entry level models. At that level giving up dial card slots, some FPS and AF performance on the D600 to save £500ish on a 6D would probabley be a popular choice.