well. ok. i don't want it to turn into a complaining about Windows thread anyway. The point is it wasn't multi-user from the beginning, and usage habits formed.
but yes. Amiga security. Basically there isn't any. Any software can write anywhere to memory at any time, and anywhere to disk, and there is not a lot you can do to stop anyone without an MMU.
However i think there is scope for "some" level of security without an MMU. It could be possible to create a filesystem with privilege access levels. Of course it would always be possible to bypass it by writing to hardware registers directly but a casual user wouldn't be able, for instance, to randomly delete system files. We could even have user accounts, with each user only being able to access their own account.
As for what we could do with an MMU, well block all direct access to disk control registers for one thing! Or at least the hard drive ones, we could, perhaps, allow trackloaders to work as long as you boot from the disk, but block that as soon as workbench loads.
Then, if we really want to push the boat out, partition "kernel" space into a different memory map. Keep user-space all in one big memory map so user programs can still interact with each other, but at least they won't be able to screw the system up, and we'd be able to detect accesses to memory that wasn't reserved at all. We'd have to keep Chip RAM flat or everything would get a bit confusing.