Various core system bits that are tightly integrated using undocumented APIs and assumptions about what the other parts are doing internally which are subject to change at any time. I think you can replace some, though not all, of the non-PID-1 bits with a little development work, but replacing the core parts is unsupported.
That is beside the point. The argument was "That's not the init systems job." to which I presented the counterargument that systemd is not just an init system.
See my comment at https://news.ycombinator.com/item?id=7639730 - those are the external APIs used by software that isn't officially a part of systemd but still depends on it. The APIs between different parts of systemd are undocumented and subject to change. In fact http://www.freedesktop.org/wiki/Software/systemd/InterfacePo... says as much itself and exists in part to convince people this isn't a big deal because in theory someone could write their own components that implement the same APIs. (Except that in practice no-one has reimplemented anything beyond the most trivial APIs, and in fact that page claims some critical stuff like logind cannot be independently reimplemented or used without systemd.)
