Also, my experiences using ZFS(onlinux) without any raid (i.e. single disk with "copies=" > 1) is that it wasn't very resilient against any kind of I/O error - even if the cable was bumped or something. This has resulted in the loss of a whole zpool and restore from backup.
Yikes. Is it similarly fragile in the event of sudden power loss?
Yes; YMMV but according to http://n40l.wikia.com/wiki/Wake-on-Lan_(WOL) everything in the Microserver family does support WoL.
Power-wise, one thing that never really satisfied me about the setup was that it was hard to spin down disks. I never really dug into why but suspect ZFS is fairly 'chatty' with the underlying storage which prevented the spindown timeout being reached. Can anyone who's looked more closely confirm or deny this?
I wrote this five years ago and it stands up pretty well. HP Microservers are a great device but if I were to build a new one today I'd look into using one of the many cheap ARM boards (NB: ZFS allegedly needs 64-bit to be stable) with disks in a USB-attached enclosure. Performance with USB3 should be enough for any home NAS and the hardware should come out even cheaper. There may even be ARM-based NAS boxes on the market that'll do the job.