I believe that the Nikon D810 is currently the best all around camera you can buy for the money ($3300) even with competition from Sony's A7R. The more difficult question comes from current D800 owners like me; "Is this camera worth upgrading to?" The answer isn't cut and dry.
The D800 was an incredible jump in technology for it's time. The D700 was 12MP and the D800 jumped to 36MP without losing the incredible ISO performance. The D800 never really had any major downfalls and therefore it has become very difficult to even imagine a camera much better than the D800. The D810 is not a completely new camera; it's an upgrade and some how they have managed to upgrade almost every major feature of the camera. The question is, are these upgrades worth selling your current D800 (for the current used price of $1800) and paying $1500 more for a D810? I'll tell you about my findings and you can make that decision for yourself.
Although the sensor is "new" we can't see a massive upgrade in terms of ISO performance. Nikon has now removed the anti-aliasing filter on the sensor and so this camera can produce slightly sharper images, similar to the D800E. Although the Canon 5DMK3 has better ISO performance, the extra pixels from the D800 and D810 can add more detail. Basically all 3 cameras can take incredible pictures and image quality should not be a deciding factor when it comes to upgrading from the D800 to D810.
Lower Native ISO
The new native ISO is 64 on the D810. This gives you one extra stop of light to darken ambient light. This is particularly useful when strobing outside. You can also extend this ISO down to 32 for even more control in exposure.
Although I have not used this camera at a wedding yet, the D810 is supposed to have far superior autofocusing to the D800. Depending on what type of photography you shoot for a living, you may want to upgrade for this feature alone. Apparently the D810 has the exact same focusing as the Nikon D4s camera. Group-area autofocus similar to Canon's is now available allowing you to choose a group of 4 focus points rather than just one or all 51 focusing points. This is helpful for capturing moving subjects when you want to maintain a specific composition.
Better Time-Lapse Features
The D810 can shoot 9,999 images in a row automatically, up from just 999 on the D800. It has a range of other features as well including exposure smoothing. I have not had a chance to test this feature; in fact, I have never actually taken a timelapse with a DSLR so you may want to read about this feature elsewhere if you are interested.
Faster Frame Rate/ Better Buffer
The D810 can now shoot at up to 7 frames per second with an MB-D12 battery grip, up from 6 from the D800. You can shoot 28, 14 bit RAW files in a row at this speed before the buffer fills up which is double the amount the D800 could hold. I've never filled up my D800s buffer before nor have I ever shot more than 2 frames per second so this doesn't really affect me.
Better, Quieter Shutter
The shutter is now much quieter and they have added a quiet-continuous mode for burst shooting. As a wedding photography this feature is greatly appreciated. You can also set the shutter to "electronic front curtain" which eliminates camera shake due to shutter clicking. This feature is only useful to those people shooting at extremely slow shutter speeds without strobe.
Better LCD Screen and Live View
The LCD on the D4s and D810 is WAY better than any other Nikon camera. It's extremely sharp and colorful, very similar to today's high end cell phones. Dropping into Live View is far better now and zooming in to while in LiveView now actually uses the pixels on the sensor to create an ultra sharp image on the LCD rather than just interpolating an image like older Nikon cameras. Focusing in LiveView is so much easier as well as filming video.
Way Better Video Quality
The true upgrades of the D810 can be found in the video functions. The camera captures ultra sharp 1080p video that we thought looked better than the D800, 5D3, and the $8000 Sony FS700 at low ISOs. The D810 was slightly better than the D800 at high ISOs but not more than a stop improvement. The 5D3 easily beat the D810 at video at high ISOs and the Sony FS700 was by far the best, creating a super clean video at even 12,800 ISO.
Slightly Better Slow Motion
The D800 can shoot in 60FPS at 1080p now, up from 720p on the D800. I really wish this camera could shoot at 120-240fps and fill the buffer and THEN dump the footage to the memory card similar to the FS700 but it cannot.
Much Better DX Video Recording
The D800 allows you to switch into "DX" mode while shooting video to get extra reach out of your lens (example: a 200mm lens becomes 300mm in DX mode). The D800 in DX had massive video quality loss but the D810 doesn't have any loss in quality making this feature actually usable. Being able to use DX mode is extremely helpful to videographers and I wish Nikon would take more advantage of the 36mp sensor and allow us to "zoom in" to the sensor more than just once.
So now that you know all of the major improvements of this camera vs the 2 year old D800, it's up to you to determine if this camera is worth the upgrade. If you're in the market for a new Nikon DSLR do not be fooled into buying the D800 for $3000 when you can spend an additional $300 and get the new D810. There is no doubt the D810 is worth $300 more for these upgrades.
Used D800s are currently selling for $1800 on eBay which means that if you own one (or 3 in my case) you will wind up paying $1500/camera to upgrade each camera. This also could make it difficult for those people who do not own the D800 but who are looking to buy their next camera. Do you buy a slightly used D800 for $1800 or do you spend $3300 and get the slightly upgraded D810? I personally do not believe that the still photography advances alone are worth the upgrade unless for some reason you really need the enhanced autofocusing, but If you shoot video a lot like me, it could easily be justified. I'm honestly still on the fence about upgrading. My D800s are incredible cameras but the new video features of the D810 are extremely tempting.