Nikon and I had talked about maybe doing a preview of this one, but I came to the same conclusion as many here - although I think it will be handy for some, F/8 really hurts it, at least for my core audience.
I think the 14 plus to -1 magnification ratio and F8 is a tall ask for low light sports action and fast moving wild life.
8 stops of IBIS and 10 stops of low light focus technology like in the ZF and forthcoming no doubt in other cameras may help this versatile tool, and defiantly allow for lower shutter speeds used or tolerating F8 a little more than usual, i do feel its also a tool more handy for video, as new mainstream users today are doing more video than stills thanks to Instagram U Tube and social media.
I am certain Nikon will harnesses some RED genetics to breed a stable of new gear or simply infuse existing gear with upgrades, i think we will see some interesting outcomes going forward, the money, growth, profits and future generations are all simply in Video and we all know this.
New IBIS technology light sensitivity with larger lens mount diameter all compliments pushing the boundary further than before, ie: F5.6 to F6.3 F6.3 to F8, F8 to F11.
I do think that the 28-400 is a great extremely versatile travel lens or a do all handy fair weather tool for video and capable to do a lot more, but is it a fit for wild life action ?
Short answer on the 28-400, for my self i don't think its a total fit for the serious wild life shooters.
Interesting times.