When rebooting a server "zfs mount -a" can take "forever" (or atleast a silly amount of time) if there is a ZFS filesystems that are at or over 'refquota' when the kernel tries to execute "zfs_unlinked_drain" (and that filesystem has unlinked stuff to drain).
This effective allows a user to cause a denial-of-service if the server happens to reboot due to a bug...
Make sure "zfs mount" is done in the background in the /etc/rc.d/zfs startup script - so that you can get a login prompt, and then give the filesystem more quota so progress can continue.
I'm closing this bug. Hopefully things will behave nicer with OpenZFS in the future :-)