I think this will be a fine camera, basically an improved D3x at a lower price.
First, there's going to be a gap at Nikon, the D700 is right now at $2600, the D800 will be $4000. I think something at $$2600-3200 wouldn't be bad to release, especially because I think many Nikon users want a new FF camera, just not with that many pixels. So either a D800, D700x that has 24-30MP with faster performance.
That means Canon has to announce a 5D Mark III and another camera, either 5Ds Mark III that could have also improved video, or perhaps a 3D/6D line to compete with Nikon's second model (not the D800).
And about file sizes - luckily it has USB 3.0 so at least at transferring it won't be so long. Editing and saving - well, that's horrible but no one is forcing you to buy the camera. SSD disks are getting cheaper (though not as cheap as most of us would want them to be), speeds are getting better and sizes are increasing.
And about lenses not resolving details - I am very confident all lenses out there, especially prime, will be able to match up to such high pixels.