You can grow it horizontally by adding a new vdev. ZFS doesn't allow you to do anything you may ever possibly want to do, but it does allow you to do more than any other RAID solution. It's not perfect but it's still pretty impressive.
At first, I assumed that RAIDZ was the obvious way to go. But I switched to a concatenation of mirrored pairs; I grow the array by adding another pair to it.
I've actually had six disks fail in six months, without data loss. Was scary. But wow.
(I'm also frustrated that it's not possible to change redundancy within a vdev — e.g. go from RAID-Z1 to RAID-Z2.)