Smaller and lighter
The body is more compact than Canon's other full-frame offerings.
You can get a better sense of the body size when it's equipped with the 40mm STM pancake lens. The sensor doesn't have the onboard hybrid contrast/phase-detection autofocus, so you wouldn't get the benefit of faster AF shooting movies with the STM lenses.
Compressed control layout
Unlike most of Canon's higher-end dSLRs, the 6D lacks the row of buttons down the left side of the LCD, which allows for a narrower body.
The camera lacks a built-in flash, which some people might miss. The buttons on the right shoulder are a bit more streamlined than the company's other full-frame models.
Unlike the D600, which can take APS-C-mount (DX) lenses and automatically crop to the frame, the 6D takes only the full-frame EF-mount lenses.
Rather than incorporate the old sensor from the 5D Mark II, the 6D uses a new 20.2-megapixel version.
The 6D uses the same battery as most of the other Canon dSLRs.
ISO sensitivity settings
You can configure the range the camera uses when set to Auto ISO.
Single SD slot
The single SD slot is somewhat disappointing; the Nikon D600 has two.
Updated mode dial
Unlike other Canon dSLRs, the 6D's mode dial has 360-degree operation, with the modes spaced out evenly around it. This is a nice change. It also uses Canon's middle lock button design.
The 6D has the usual set of connectors -- wired remote, mic input, USB, and mini-HDMI -- but some video shooters might miss a headphone jack.