Q: Why can't a Storage Pool created in Windows Server 2012--now hosted by Windows Server 2012 R2--use tiering or the write-back cache?
A: A Storage Space created in Windows Server 2012 isn't automatically upgraded to the new Windows Server 2012 R2 Storage Space version. To enable the tiering and Write-back cache capability, you must manually upgrade the Storage Space to the Server 2012 R2 version, which will mean it can't be attached from Windows Server 2012 anymore. Here's the command to upgrade:
Update-StoragePool -FriendlyName <name of Storage Pool>