The GFX 100 offers capabilities that no camera has offered before, by bringing the most current sensor and stabilization technology to a larger size... Roughly speaking, it should have around twice the image detail of a modern 24x36 mm pixel monster, 2/3 that of a Phase One IQ4 150, and four times that of the best APS-C cameras. Other than the Phase (which lacks any form of image stabilization), they all offer closely related focus and stabilization capabilities.Due to the stabilization, the Fuji is handholdable under many circumstances - it's also weather sealed, which the Phase is not.
Everything from the best of APS-C to the IQ4 150 now uses similar sensors, just cut to different sizes. The GFX and the Phase also offer 16-bit readout, which should make a slight difference in dynamic range now (and it will become more important with future sensors). Except for the Phase, which is disproportionately bulkier, more difficult to use, and more expensive (5x the price of the GFX for 1.5x the sensor), there is now a somewhat reasonable relationship where you pay more, accept more weight and get a bigger version of a similar sensor.
The question it all raises is "how do we display these images"? Even a great APS-C camera can make an image indistinguishable from the GFX in nearly any internet application. The 24x36 mm pixel monsters can fill even an 8k display (the Nikons and the S1r can for both definitions of 8k, while the Sonys do as well for the usual definition, but not the cinema version), and make 24x36" prints and above. The display format for the GFX seems to me to be 40x60" prints viewed up close. How common are opportunities to display such prints? They exist, and the GFX 100 offers by far the most versatile way yet of capturing the images needed, which had previously been the province of $50,000 digital backs and large format (even beyond 4x5") film.