Photographically speaking not a big leap AFAIA... +4MP, possibly extra stop of low light sensitivity - that's it... It seems that pro cameras are reaching a sort of plateau...
Bet you the colour rendition, saturation and image look and feel is a step-change improvement. That's what stood out for me in the change from D2X - D3, which my friend the wedding smudger down the lane uses.
I'm hoping they will stick at DX format for a D400, otherwise I'm going have to invest in some new glass when I upgrade the D300.