Peter Bright at Ars Technica:
Unlike RAID systems of old, but in common with other modern storage technologies such as Solaris' ZFS and Linux's btrfs, pools can use disks of different interface technologies—USB, SATA, Serial Attached SCSI—and different, mismatched sizes. New disks can be added to a pool at any time. Pools can also include one or more hot spares: drives allocated to a pool but kept in standby until another disk in the pool fails, at which point they spring into life.
Storage in a pool is then distributed among one or more spaces. Each space can have its own redundancy policy, with three kinds of fault tolerance offered: 2-way mirroring, 3-way mirroring, and RAID 5-like parity. With the mirrored options, a space's data is stored either twice or three times within a pool. With the parity option, the system will compute additional information and store this within the pool. If any disk in the pool fails, the data can be reconstructed using this additional information.
A feature of both server and desktop Windows, this is killer.
If the feature does indeed ship in desktop Windows, it will overnight obsolete a range of SOHO-oriented storage systems; products like Drobo and ReadyNAS will find it hard to survive in a Windows 8 world.
It looks like I'm going to have to build myself a Windows 8 storage server.