I'm hoping for btrfs to appear on my NAS. The reason is it allows for snapshots, which are basically backups of what is stored. These take no space; only data updated takes a bit of space. Thus, an MP3 that is tagged will only consume 4k or so extra to keep the two versions. This way, many backups can be kept on a relatively small disk. Just make a snapshot every now and then, and when space gets tight, delete an old snapshot.
It will also be possible to make a backup of a computer to this using rsync. With the --inplace option, rsync will write only modified sections in files - and btrfs will consume only space for those blocks. Allowing efficient backup of a computer.
Apart from this question, I wish to make a great thanks or 'heel erg bedankt' for making the Network Space V2 lite a usable device :)