After a morning's shooting with it doing my usual livestock show stuff all I can say about the D780 is that it has more changes than I expected and is far better than all the reviews said. reviews which made me think the changes were marginal. For me, they are not. The difference in the liveview performance and operation between the two cameras is night and day. The touch screen is great. And there is face detect through the viewfinder - not something I need too often but it's already been usefu. For me all those features are really useful. It's also noticeably quieter - even in Quiet Continuous (which is how I usually use my D750s). The screen is much clearer, which isn't a big deal, but is nice to have. There's also a review option to show only the image with no info around it at all. I love that. Oh aye. If using one of the crop options the bit of the viewfinder not being used is greyed over. That's much better than the 'box' in eh D750. There's a square crop which is nice, but no 5:4 which would have been nicer.
Biggest drawbacks are that it won't recognise my cheap batteries (but my genuine Nikons are fine) and Lightroom 6 won't read the raw files.
I'm glad I took the plunge. Not so glad that I'll have to subscribe to Lightroom, buy a second body, and get some genuine Nikon batteries! I'll be keeping my most battered D750 as back up,because it won't be worth much, but mostly because I know I'd have seller's remorse.