Reiterating much of what has been mentioned... I've outgrown my D90's capabilities and was looking to the "D400" for: - higher FPS for sports and wildlife - better low light performance - DX crop factor reach for wildlife - preservation of investment in some good DX glass I already have - use of DX lighter weight lenses for long photo hikes - increased number of focus points - improved dynamic range capture - improved image quality - etc....
Would I like a full frame body? Sure, and I see one in my future at some point. However, to make that work it means at least one or two new FX lenses.
My roadmap, as I see it is:
D90 -> D400 -> FX mid range lens such as 24-120 or 24-70 -> D600 or D800 -> FX wide lens
Updated post: clarifying my roadmap, I see the FX body serving a different purpose than the D400 and not as a replacement for a D400.