Hmmm. I run NTFS and truecrypt on top of raidz zpool allocations (zvols, virtual block devices) all the time, block storage doled out over iSCSI from my ageing Nexenta box.

I do think you're too quickly dismissing the advantages of the fs being aware of the underlying block storage provider, to the point of being able to grow elastically from common storage rather than preallocate at partition time.

I'd much rather the whole md layer be replaced by zpool even if zfs itself is not used.