Nikon improved the grip for the D810. The D800's is fine, but you don't realize how much better the D810 feels until you compare.
In addition to getting a new Flat Picture Control that opens up midtones, you can now control the settings in 1/4-step increments.
No changes in terms of ports, although the button for the autofocus modes now has a different texture.
The Nikon D810 retains the CompactFlash/SD card slots of the D800.
The metering switch that used to be located around the AE-L/AF-L button is now a button elsewhere.
The metering button now sits above the release-mode dial.
The i button, which provides access to some less frequently used settings, migrates up from lower-end models.
Everything on the top of the camera remains unchanged.