To be precise, Windows 3.x did multitask, at least on a 386 - however, only on the much more primitive, cooperative level (which the application needed to allow and some just didn't). Amiga OS offered preemptive multitasking from the beginning - a feature which required 10 years to reach the mainstream Windows platform (Windows 95).
It would preemptively multitask DOS applications, just not windows applications until Windows 95. Even within Windows95 I believe all applications shared a common message queue, which was a point of failure for smooth multitasking.
Anyway, there is nothing really magical about preemptive multitasking in just 256K of ram(or less), and if you had a A1000 with just 256K of ram, you probably were not doing much multitasking anyway...
The preemptive multitasking was just one of the many cool features. The GUI, and API and custom hardware were equally important.
Cinemaware games were a good example of what could be achieved in the OS (with the hardware) without banging on the hardware directly...
