I received my D800 a couple of days ago...and the thing that strikes me as most advantageous so far is the ability to crop a high resolution image when necessary. Here's an example of a shot from yesterday:
This image was a quick grab. I noticed the African Wild Dog beginning to open its mouth, so I turned and quickly captured this image. The shot isn't particularly well composed. And, I couldn't zoom in any closer as I was using a 500mm prime.
I then cropped the image for what I hoped would be a stronger composition--making my subject larger and focusing more attention on the subject and his teeth! This isn't the greatest photograph even after cropping...but my intention is to show the power of a larger file.
After cropping, I still ended up with an image that measured 4,736 x 3,789 pixels. That's more pixels than a full sized file from my D700!
Of course, it's best to compose the photograph properly right from the start in the camera. But, when that's not possible, it helps to have this added flexibility after the fact during the post processing.