Keep in mind that this is not a complete new camera body from Nikon. It's more like trading your 2013 model Honda Civic in for a 2014. There are differences, but overall it is still a Honda Civic. The Nikon D800 is a full frame sensor camera with an outrageous megapixel count of 36.3MP. There will be no change to that in the Nikon D810 but there will be a change to a new processor. The new D810 will have the EXPEED 4 processor much like it's bigger brother the professional Nikon D4s which will be a faster processor. Not only that but you will apparently get better noise performance and the Nikon d810 will have an improved ISO range from 64-12800 making it a lot better in low light shooting. On top of that you will get things like a new metering system, better AF, and 5fps continuous shooting. One other thing is there will now be RAW size S which will basically be a smaller compressed RAW file.
On the video side of things the Nikon D810 will be sporting 60fps in 1080! This means you can now record in full HD and create some nice slow motion shots. For all you videographers out there, this camera has a lot of small tweaks you may be interested in and I will provide a link to see the comparisons below, one of which is zebras which is handy.
Other small but significant updates are a bigger better resolution back screen, the same battery but more battery life(Not sure how that works.) and many other little things.
All those small improvements are nice but the news for me is that the nikon D810, like the Nikon D800e, will have no OLPF. This means that you will no longer have to choose between the body with and the body without the OLPF which was kind of weird from the start seeing as how removing something that effects image quality actually sets you back another $1,000... So you were basically paying more for less. For those of you who do video you know that the only real option is no OLPF to eliminate the moire issues.
Seeing as how, once I am back in the states, I will be in the market for a new camera, the question now is, "Will you consider this camera?" The answer is, "No." The camera is going to be great. I have seen, held, and tested the Nikon D800e before and it is a beautiful camera. This Nikon D810 can only improve on something that was already great. But realistically, even if I could afford it (which I can't at $3,299), I just simply wouldn't need it for what I do. Everything about this camera screams "studio camera" to me and that just simply isn't what I like. I have worked for a studio before and I just can't get into it. There is a market for this camera, and there will be those who need what it offers, but for me I just simply don't. However, if you do need a high megapixel count, no OLPF, a nice ISO range, and 60fps HD video, then this is probably the camera for you. As for me, I really like the Nikon D600/610.