The lack of documented software that used them to enable useful (to me) functionality.

I was using some of them, such as kvm for my virtualization and lvm for disk management. But systemd still had a substantial 'oh, wow, Linux lets process management be this easy and powerful?' factor, showing me something new that I hadn't seen in my use of any other system (FreeBSD, OpenBSD, Windows, a touch of Mac).

the documentation directory of the kernel source is actually pretty nice. theres a bunch of utilities for things like cgroups, namespaces, etc. they're not well known but they work perfectly fine.

I suspect its not well known because there was no commercial, marketing drive behind them. Nowadays at least one of these seems to be needed to even gain visibility. People don't go search what's cool/good where it is. They wait for HN or some other news website to tell them

Just like the regular news really. Turns out it doesn't work all that great.

Setting up and running services like the OS does, but as non-root user, is kind of a big deal for my use case.

Perhaps I'm ignorant and this was always possible.