With all the information about the D800 having leaked in advance, it's easy to tell what's been most attention-grabbing: the high-resolution sensor and the D800E sibling model which incorporates a modified low-pass filter system that results in little to no antialiasing. There's no doubt that the combination should appeal to professionals like studio and wedding photographers.
However it's notable that the sensor's pixel size is 4.88 x 4.88 microns (compared to 6.4 x 6.4 for the 5DM2) and hits a comparatively low maximum sensitivity of ISO 25,600. That said, cameras for this target market don't need the really high, gain-pushing ISO sensitivities of more action-oriented models; they need the highest clean setting. And Nikon has a history of clean high ISO images for its pro models. But even if the D800 manages impressive video, I suspect that the D800E will be less video friendly--aliasing can be a real problem in video and it's much harder to correct in post-production, so you need that low-pass filter. Medium-format cameras and AA-filter-free models like the Fujifilm X100 usually don't support video or don't produce professional-quality results.
With the exception of the sensor, the D800 uses a lot of the same core components as the D4, including the viewfinder, autofocus and metering systems; capabilities enabled by the Expeed 3 image processor like improved white balance, face detection boost for exposure and dynamic range optimization and in-camera HDR; and the same set of enhanced expanded (for Nikon) set of video capabilities, including uncompressed 4:2:2 HDMI out, mic and headphone jacks, live aperture adjustment while shooting, and expanded set of supported frame rates. Nikon also claims the fast sensor readout signficantly reduces rolling shutter artifacts. As you'd expect, it also has a magnesium alloy dust-and-weather sealed body. To me, a couple of the subtle standout features are support for USB 3.0--essential if you shoot tethered with those large files, for example--and two card slots.
While it sounds good on paper, Nikon still has a lot to prove when it comes to video. I haven't been awestruck by the samples I've seen from the D4 compared to various Canon models and the (admittedly expensive) interchangeable-lens camcorders. (Speaking of which, Nikon plans to post a D800 video sample tomorrow.) But the rest of the package sounds quite nice. Plus, Nikon may keep the D700 around for a while, and if the price drops a few hundred it would be an attractive option for the video-indifferent folks.
The real wildcard at this point for buyers thinking about jumping to or from Nikon is the update to the 5DM2, which is likely to be announced within a few weeks, in time for WPPI, a big show for wedding photographers at the end of February. Current rumors spec it at 22 megapixels with the same autofocus system as the 1D X, among other things. This is shaping up to be one heck of an interesting year for pro cameras. No complaints here.