To me, it looks like the big news is the new mount enabling better lenses.
The fact that it is mirrorless, doesn't make that big of deal to me. I have a Nikon D750 and if I compare specs to the Sony full-frame mirrorless, there is barely a weight difference.
I like the battery life that I get out of my DSLR as the sensor doesn't have to be always on and I can walk around with it and not worry about the battery.
I have heard people go on about the electronic viewfinder being better in some instances. I don't know.
When I think about it, I mostly think about the lenses, though. It'll be fun to see what lenses Nikon brings with the new mount.