Every time something to do with Plan 9 comes up I lament the fact that the most elegant elements of its design have failed to be realized in practical terms in other operating systems. When they are realized, they usually lose the elegance.

Mostly I think about union mounts (which are very different in plan9 than the version that's been integrated into Linux) and the privilege escalation system (which has always seemed, to me, to be so much better than the sieve that is setuid).

(Note: I'm aware there have been attempts to bring purer versions of these things to linux, but they never gain any traction)