Yes, the AF-ON button is a top requirement and do miss it on the D7100. Re-assigning other buttons is not something I'm keen on doing. Once again the fps is restrictive on the D800, but on my D7100 I find it ok, buffering however is not good. The 10 pin connector is very useful as I have the MC-36 intervalometer and use it on the D800, but the remote for the D7100 is very usable.
So, there are pluses to a D400, whether the price is going to be too high to give these extras I don't know. I am saving for the 80-400mm AF-S and that's now my priority when the price drops.
I love both my D800 and the D7100 reduced my desire for D400