Upload & Sell: Off
I can't speak on the business reasons for releasing or not releasing a D400 or even if it would be a viable product, but I think it would interest me.
Owning the D300 and D700, I find them to be an excellent pair of cameras for what I like to shoot. Sometimes the smaller sensor lets me get just a little more reach that I need for the shot without having to go for bigger glass. Even a 600mm lens won't give me all the reach I need, though admittedly the longer the effective lens length the more difficult it is for me to get a clean shot. Still, I like using both FX and DX and love the feel of the D300/D700 bodies. I may go for a D7100, but if a D400 was available I don't think it would be a difficult decision for me to go with the better body.
To be honest though, most of my shooting lately has been with the Canon G11 or G1X. No SLR will ever give me the portability of a P&S, even if they do have the ability to blow them away in IQ. The fact is that I'm just an amateur photographer that enjoys decent gear, but struggles to justify every new advancement that comes out. Often a nice P&S covers what I need if I am not specifically going out to do a photo shoot.
For DSLR shooting, 12MP is plenty for most of my shooting, but I wouldn't mind having more for times that it would help. I think 18MP would be more than enough for me, 36Mp might be way more than I could use. I wouldn't mind seeing a D800H that has a lower pixel count on the sensor (16-24MP) but I think that is essentially what the D600 is supposed to be so I'm not holding my breath.
So yeah, a D400 would be a camera I would purchase. Especially if they kept the price close to what the D300 was released at (~$1800, if I remember correctly.) Everyone has their needs, and for me a D400 with a 16-18Mp sensor would fill a desire I have in my camera lineup. If not, the D300/D700 lineup works fine for me until I can justify the high cost of a D4, but I think I would have to be doing a lot more shooting than I am now to get there.