I'll be curious to discover the feature set of whatever full-frame camera ends up being released in this Nikon product slot.
DX crop - the D3 has this, but it seems like something that might be dropped in a lower-end camera. That'd be too bad, since it'd be nice to have the option to use existing DX lenses (okay I realize there's nothing really stopping you anyway, but getting proper exposure, framing, etc. might get a bit more involved). So I'm really hoping this makes it to the "D700".
Overall FPS - for pro photogs this can be important; and the companies all seem to use it as a big marketing point. Personally, I don't care all that much if it's 2.5fps or 8.
Pop-up flash - I've heard arguments that a full-frame camera won't have room for this because of the larger pentaprism required. The Canon 5D doesn't have one, and Nikon's film SLRs that share this body type don't have one either. I've got an SB-600, so it doesn't matter that much; but sometimes it's handy if you're going light (say if you're just taking photos at a family birthday party).