[senko]

It's at the same time pure microkernel and performant and usable in real world (not just a research project).

The memory management, scheduling and IPC are implemented in an interesting and novel way (compared to your typical mainstream OS) and allow for quite different OS design.

In particular, capability-based IPC and memory management could provide a robust base for a secure OS from the ground up, instead playing whackamole in an aging POSIXish design.

Sadly (tho I understand the rationale), most approaches to building a full OS on top of (L4 in general) are "run userspace Linux in an isolated container", which kind of ignores all the power underneath.