I think there are three main areas that are substantially different to the the X-Pro2 that I, at least, have been testing and appreciating the most:
The Tilting Screen
As someone who shoots weddings for a living, I know the power of tilting screen. Whilst shooting with the X-T2, I've found myself using the now vastly improved tilting screen a lot.
The new screen will pivot in two directions enabling it to be useful not only for those shooting horizontally, but also those shooting vertically.
This is something that will either excite you, or pass you by completely.
It excites me, and I intend using the 4K features on personal and social documentary projects as much as I possibly can.
There is, of course, a swathe of cameras with 4K capability out there, but none that allow you to film natively using the remarkable colours and rendering of the ubiquitous Fujifilm film simulations.
You will, come the time of camera release (we haven't been able to during the beta testing) record to a F-Log flat uncompressed output too. This is perfect for those that have their own cinematic finishes they like to apply during colour grading.
Fuji have added a headphone socket to the Vertical Grip and also a 3.5mm mic jack to the body. Along with an HDMI out port, this means the camera really is geared up for some serious filming for those that are inclined.
Here are some random clips shot at 4K, straight from the camera. The Colour clip is Classic Chrome and the monochrome ones are Acros+R. Sound is recorded into an Rode Shotgun mic.