AmigaOS never implemented any memory protection, did it? So it hugely favours a microkernel design, since there's no expensive context switch to translate a call into kernel mode; everything is user mode.

Whilst you are correct in that AmigaOS didn't implement memory protection, it's possible to argue that wasn't by design.

The original design for AmigaOS was known as CAOS. The CAOS design included resource tracking, which I believe is a useful feature when it comes to implementing memory protection.

If I remember correctly what ended up happening was the microkernel for CAOS was developed in-house (Exec), but contractors were being paid to develop the rest of the OS, and they disagreed with the design decisions of CAOS and wanted to make it more Unix-like. Once the Amiga team realised the contractors had wasted time developing something different from what they intended, they sought out an alternative solution to replace the user-space side of the OS. As a result they contracted a different team to rework an OS called TripOS to use the Exec microkernel (that was originally intended for CAOS), which then became the AmigaOS that Amiga users know.

You can read a bit more here:

http://www.amigahistory.plus.com/caos.html

Quark microkernel in MorphOS https://en.wikipedia.org/wiki/Quark_%28kernel%29

MorphOS - quite fast and usable :) http://www.morphos-team.net/intro

So, out with the old and in with the new.

Does MorphOS have resource tracking and memory protection now? It didn't last time I was following it (about 3 years ago). To be fair, none of the next gen Amiga operating systems (OS4, MorphOS, AROS) had implemented that last time I checked.

The second paragraph of the Wikipedia article mentions user/kernel-mode division and kernel-protected objects. Past that I don't know. Probably none for legacy systems and similar to Darwin (coarse-grained) for new ones. Could be more fine-grained, though.