The two things that the 6D is lacking that I think are important to a amateur/prosumer photographer are a second memory card slot and a pop-up flash. I just purchased a Nikon D600 for my father, rather than the 6D, because the D600 had the second memory card slot and a pop-up flash.
I consider this rather as a feature: it saves weight and allows a more compact camera design. I prefer to shoot with a remote flash or with lower stop/higher ISO.
And as for the thread-topic: the 5ds don't have build-in-flash either.
