There is something refreshingly Nikon-like about the D810 refresh. Even though there is nothing earth-shattering about the technology, there are a multitude of changes throughout the camera that synergistically add up to an improved shooting experience. Though it is true that much of the sensor-level improvements relate to video work, the overall stills photography experience with the D810 is also improved... and in a way that does not easily come across when you run down the spec-sheet.
The most immediate tactile sensation with the D810 is the re-profiled grip. For most people the D810 will be slightly easier to hold than the D800 because of a deeper notch for your middle finger on the front of the camera, an extended thumb ridge on the thumb-rest on the back of the camera, and a slightly coarser texture to the rubber all around the grip. The D800 wasn't a difficult camera to hold, but the majority of people will appreciate the extra bit of hold that the new grip shape affords.
It's not just comfort; that translates into practical benefits as well. When paired with the AF-S 24-70mm f/2.8G, the new grip shape makes the big standard zoom feel a little less cumbersome on the D810 than it does on the D800. From a product design standpoint, this is probably a smart design choice on the part of Nikon, as even though the D810 is clearly a refresh of the D800, the tactile sensation of holding it gives the impression that the D810 is its own camera and not derivative of its predecessor.
Another nice (but not truly necessary) change is the ridge separating the AE-L/AF-L button from the AF-ON button. There was never any danger of confusing the two on the older camera, but fans of back-button focusing will like the physical separation between the two buttons. This works well when you are shooting in a hurry; your thumb sides from the control dial/rest area over to the AF-ON button and the ridge gives you tactile feedback on where to stop.
The change in location of the exposure mode button takes some getting used t, as it is now on the left control pod instead of the traditional pro-layout of being a ring dial around the AE-L/AF-L button. It takes getting used to, but from an ergonomic standpoint, it is an improvement. You couldn't switch the exposure mode on the fly with the old arrangement (that is to say, with the camera held up to eye level) and the new arrangement brings the control in line with Nikon's pervasive theme of "press-and-twirl" operation. The bracketing button re-location has forced a slight-shift in button placement on the left side of the prism-housing. It's not a radical change, but it will force long-time users to re-learn some of their accustomed muscle-memory.
Though something that was easy to shoot around, the D800 was notorious for people complaining about the green tint of the rear LCD screen. That is now customizable. This in combination with the higher resolution of the screen (1,229,000 dots) does make for modest but perceptible improvement.
In real-life use, the overall experience of the D810 is that it is familiar, but slightly better in most ways. Even though there is now an electronic first curtain option for quiet live-view shooting, the basic design of the new shutter mechanism is noticably quieter than on the D800, and makes the D810 one of the quietest bodies in Nikon's DSLR lineup. Autofocus seems to be a bit more reliable than with the D800; that's not saying much since a properly tuned D800 was no slouch. There's a new "Group-Area" focus mode that is inherited from the D4s. This autofocus mode uses five points in a cross; you lock focus with the middle of the cross and the camera uses the remaining four points to track motion. How is this different from 9-point focus mode? Nine-point mode seems to be more heavily weighted towards the center point than Group-Area, and works better for anticipating erratically moving subjects. The new focus mode was designed specifically for deliberately tracking moving subjects and to reduce the change of the focus point falling behind the subject. Though it's hard to quantify outside of a controlled test environment, Group-Area does seem to produce more "keepers" than 9-point.
This is just a small aspect of using the D810. Added together with the video and image quality improvements, it is a satisfying step forward for Nikon. Unlike the D610 launch, which felt hollow and did not resonate with consumers, and the Df, which felt like a marketing experiment, the D810 focuses on the core virtues of usability and image quality. It's not enough of an upgrade to justify an immediate purchase for existing D800 owners, but there are enough improvements to make those users wish that the price was a bit lower to make a switch easier on the wallet.