The best thing libvirt[1] does is to have a stable API from day 1. Over 7 years and counting. In practice this means a couple of things: We have to be careful and considerate when introducing any new API, since we're going to be maintaining it for a very long time. And although we do deprecate APIs occasionally, we keep them around forever so programs don't need to be "fixed" all at once (or maybe never).

[1] http://libvirt.org