I'm also having this problem. I'm able to reboot my laptop (even though there is a bit of a delay), but my computer never powers off (unless I hold down power button).
I've tried adding both 'reboot=bios' and 'reboot=bios' to 'GRUB_CMDLINE_LINUX_DEFAULT' and neither one has any effect. I also tried 'acpi=off' which was suggested here. Unfortunately, this didn't resolve the shutdown issue either.
I'm currently using xubuntu 13.10
$ uname -ior 3.13.0-031300rc8-generic x86_64 GNU/Linux