It's really a matter of how you line up the features. I placed an order for a D800 with no specs more than a year before its release. I was looking for a D700 upgrade - and pleasantly surprised with the D800. The added pixels of the D800 are a big positive for my style of shooting. The improved dynamic range, color rendition, and some of the other features are positives. My first images from the D800 were simply stunning - a completely different look than any of my earlier cameras.
I've been using the D600 for about a month trying to give a chance for any issues to be worked out before ultimately converting it to IR. I want a full frame IR camera and the D600 is superior in several respects to a D700. Some of the key positives are twice the pixels, better dynamic range, better color rendition, and a lighter body. And it has a movie capability that is increasingly important. The list price of the D600 is 30% lower than the D700 was before end of life discounting.
The controls of the D600 take some getting used to. The body is slightly larger than a D7000 - enough to matter in a positive way. There are some control changes with the smaller body footprint. I don't really care for the ISO button being moved from the top. But the AF controls are better once you get used to them - you can see the settings though the viewfinder so changes can be made faster on the fly. This feature is common to the D7000 and D800. It just takes getting used to the new layout.
There is one other advantage for the D600 that could be relevant for IR conversion. The D700 has an IR light that is used to ensure shutter speed accuracy. This light was causing some long exposure fogging of IR images on a D700. The D600 has corrected that issue.
Based on my experience with the D600, it's a very good camera. Nothing I have seen takes away the fact that the D700 is also a very good camera - and there are some specific areas where the D700 has advantages over the D600 (build and frame rate). But my experience in handling the D600, D700, and D800 is that the two newer X00 cameras are very good and represent upgrades from the D700.
Obviously there is still a gap for those who want a high frame rate. A sports and wildlife camera with 8-10 fps would be a nice move. The challenge here is whether you want a DX camera for the crop, or an FX camera - and how big will your images be. Either way the high frame rates are going to be limited by file size and you have a tradeoff to make. I could see a 16mp camera with 9-10 fps.