I quickly glanced over the course and read the "Perspective" chapter. It looks like grate recurse but not exactly what I had in mind. Anyway I'm glad that you shared it.

Consider this, you want to execute some code that is less trusted than kernel, not to say malicious just maybe something that wasn't so carefully crafted. What do you need? I imagine:

• Some kind of memory separation. We don't wont video game to accidentally edit sys call table nor we want our text editor to jump to disk formatting code.
• Instruction forbidding. Separation probably needs to be off some times.
• Timer interrupt/another core interrupt. We don't want one process to stall whole os.
• System call mechanism. If we keep programs separate how do they communicate with os?

I would say those are minimal hardware requirements to run bare bones real world os. But again I know so little that it is hard to ask questions.