While its most obvious improvement was in its graphical capabilities, the Mark II also included an internal 5.25" floppy disk interface as well as two internal expansion slots, doing away with the need for an "expansion box" to permit upgrades.
While the price of the PC-8001mkII was comparable with other 8-bit computer offerings at the time, its graphics were notably worse than its competitors (most likely an attempt on NEC's part to avoid competing with their own, higher-powered PC-8800 series).