Read all about the D800 as compared with the 5DmkIII in this article by Dan Chung. Very interesting read and shows that Nikon had definitely arrived on the dSLR video scene. But there are a few things they could still improve, notably moiré.
There is just one example I want to single out from that article. Listen to the audio and tell me that you don’t agree that this is really, really clean and detailed.
A short video shot in one take with a production Nikon D800 and 14-24mm f2.8 lens. Not a serious test.
Featuring Grey Matter the beatboxer, BS (MC), Dan Cary (Singer), Spencer Beatbox (beatboxer/squeegee noise), Cocoa Sarai ( Singer ).