Portability

Portability within OpenBSD is very good :-)
Portability to other OSs... Pretty good, but getting harder
At least some version of PF runs on all major BSDs
Ported to Windows (CoreForce)

The project's policy here is the same as for OpenSSH: we will not complicate the base code with portability goo.
Newer performance improvements rely on PF's tentacles getting into other subsystems.