The D600 is both faster and slower than the D800. It can shoot up to 5.5 frames per second, compared to the D800's four frames per second. But the fastest shutter speed has been reduced from 1/8000 of a second to 1/4000. Flash sync speed has also been reduced from 1/250 to 1/200, which may or may not be a concern to you depending on how you work with flash.
Another thing which has been reduced is the number of focus points, 39 (with nine cross-type) compared to the D700 and D800's 51 (with 15 cross-type). 39 is a flexible number but the AF points are bunched in the center, just missing the intersection points formed by the rule of thirds. We found ourselves adjusting our composition to match the camera's clustered AF points, often shooting with a little more headroom to compensate.
Auto-focus speeds are fast and accurate, even in dim places. The D600's 2,016-pixel RGB sensor is not as rich as the D800's 91,000-pixel sensor, but metering seems accurate in most situations. It does appear a little unsure, in sequential images we could see the D600 hold the same metering in most frames but jump exposure settings in some. It also seems to prefer to underexposure in high-contrast scenes, so it pays to check your histogram after shooting.
A number that has been increased from the D700 is the number of pixels; the D600 has twice the resolution at 24MP, which is slightly higher than the higher-spec Canon 5D Mark III's 22.3MP. 24MP will gain you larger print sizes and finer detail, while being more forgiving in terms of focus accuracy than the D800's 36MP. They will also cost you more in terms of storage, a high quality JPEG image measures anywhere from 10 to 17MB.
The D600 does a great job at suppressing image noise while retaining image clarity. ISO 3200 images are clean and usable straight out of the camera, we'd shoot quite confidently up to ISO 6400 but some pictures might need a little bit of chroma noise reduction. ISO 12,800 is an option in dire situations but there is noticeable detail loss, while the maximum limit of ISO 25,600 is best avoided.
The latitude in the D600's images at low ISO settings is quite astounding. The D800's dynamic range allowed you to create almost HDR-like images from a single shot, and we're seeing the same results from the D600. High amounts of detail are retained in the shadows and highlights which can be pulled out in post (of course, these must not be completely blown in the first place).
These first results are from JPEGs, we imagine that raw files will provide even more elasticity. Raw files can be saved in 12 or 14-bit, but there is no uncompressed option, only lossless compressed or compressed.
Finally, the D600 comes with capable video specs. It shoots full HD 1920 x 1080 videos at 30p, 25p, or 24p, with a maximum bitrate of 24Mbps. 60p is limited to 1280 x 720 but you can record uncompressed footage to an external recorder via HDMI. Simultaneous display on an external monitor is also possible through HDMI, and the D600 comes with separate 3.5mm microphone and headphone jacks.