A quick browse of Anandtech.com shows a nice dissection of WHS Drive Extender v2.
The review goes into changes from Drive Extender v1, the way NTFS and the storage pool interact, 1GB file chunks distributed across hard drives in a Raid 0 fashion, 2 bit ECC, application compatability, and even a comparison with ZFS.
See the story at Anandtech.com here.