The original Amiga platform had no MMU, hence no support for memory protection or virtual memory addressing. Some Motorola processors included it (the 68030 and up) but these were only used on high-end Amigas that were not the most common platform, and are more comparable to contemporary workstations. The AmigaOS just used the single address space approach, that's not necessarily incompatible with protected memory.
The Intel 80386 and its successors were quite exceptional in being "killer micros" with a workstation-class feature set for the time.
