So, it seems that for a lot of people using unstable, hardware-related permissions (shutdown/reboot, suspend/hibernate, devices mount/umount etc.) have been broken since some times.
That's usually the case for people using GNOME with lightdm display manager, Xfce with either gdm or lightdm.
It seems that recently, policykit (which is used by GNOME and Xfce) switched from consolekit backend to logind backend (yeah, systemd-logind). So applications using policykit needs to handle that correctly, and that means beeing sure a logind session is correctly setup, which is done by installing the package libpam-systemd.
For now, it's still possible to not switch to systemd as init system, by installing the systemd-shim package before libpam-systemd. Be aware that (at least with the current state of affairs), this is only true with logind before 204. When systemd maintainers start transitionning to a later version, only systemd-sysv (so, systemd as init system) will work.
For people reluctant to switch to systemd, they can use systemd-shim for now. Then when systemd 205+ enters the archive, either lose those hardware permissions, or try to improve systemd-shim to handle that situation.
There's not much we (Xfce/LightDM maintainers) can do about that.