After learning of the existence of a bunch of compatibility layers, I'm now occasionally pining for a Linux→Mac one. Because dealing with BSD software, aka ‘unknown option --help’, is a source of irritation sometimes. Plus there's actually open-source Linux software that doesn't have open-source analogs on Mac (not that I'll remember right now what it was).

But then again, Linux has /proc and a boatload of common APIs beyond kernel calls.