The 68k was unique in that it included in its design the ability to act as a Harvard Architecture processor (where instructions and data have separate signal pathways rather than being together in the same address space [1])
It did this via the function code pins, which signaled differently depending on whether the chip was requesting instructions or data, in supervisor or user mode. No other consumer-grade chip had this as far as I know.
Sega's System 16 was also 68000 based (as was the Megadrive/Genesis), but I don't know anything about their protection scheme.