In terms of its physical design, the Nikon D810 is even better than ever. Its hand grip is noticeably more comfortable, and its tweaked control layout is very well considered, too. The new i-button, in particular, means you'll spend less time fiddling in menus, and more shooting great photos. And whether you shoot through the viewfinder or in live view mode on the rear-panel LCD monitor, there have been some very worthwhile improvements.
In late 2012, Nikon took its camera line to a new resolution high with the D800 and D800E, a pair of closely-related cameras based around a 36-megapixel full-frame image sensor. As we said in our reviews at the time, both cameras impressed mightily, and they went on to be very well-received by photographers seeking the maximum possible detail from their 35mm lenses. The Nikon D810 has some pretty big shoes to fill, as it replaces not one but both of those earlier cameras with a single model.
The D810 also has superb facial recognition so that it automatically focuses on the nearest eye, something the D610 can't do. I find this critical for photographing moving people.