camera/camera'sActually when you do a product comparison between the 8 Plus and the X besides the OLED display the only feature differences seem to be camera related. Other than that the phones are nearly identical.
https://www.apple.com/iphone/compare/
Color design(silver with black front is very nice I've been asking for black front with lighter color for a long time)
Screen is much much better(its the main thing honestly as its what you interact with-It matters the most)
Stainless steel band(another quality feeling upgrade)
Form factor(coming from a plus model thats been hard to handle at times over the past 3 years it is a huge feature)
Those reasons alone make it worth 100-150$ more