I'm replacing my D700 with a D7000 to get back the advantages of the DX crop factor (or whatever you want to call it) as well as continued ISO range with slight improvement on the low end, and 16mp. Lighter weight is also a plus. I
'll keep my D3s for the wonderful things that it does, and would have preferred to just add the D700, rather than replace the D700.
I would buy a D4 today for the 16MP, widened ISO range on both ends and the improved AF capability, especially in low light, but it's not in the budget right now. The other changes in the D4 are just adjustments that come with any new body, at least to me.