This has inspired me to re-read Melinda Varian's "What Mother Never Told You About VM Service" and it's still a magnificent document. I once again find the control files confusing as hell, but once you get used to how they work, which once upon a time I was, you had a repeatable (and unwindable!) service process.
I miss the casualness with which you'd build a new CP nucleus and test it out on a second-level system. It's so much better than anything in the Unix world, far more elegant than testing kernel patches in a Linux virtual machine, largely because of the ease with which you can attach minidisks to a first, second, or whatever-level system. I guess cgroups and bind mounts finally get you most of the way there in terms of mounting arbitrary storage to virtual systems, but it's still a pain in the ass to test multiple kernels.
Not that I spend much time anymore that far down in the system (any system!), but...VM got a lot of things right.
Adam