Essentially, the new Mac Pro moved all the mass storage except for the initial SSD mass storage to external devices, connected by the Thunderbolt 2.0 connection running at 20 gigabits per second data transfer rate (which is over three times faster than the 6 gigabits per second transfer rate of Serial ATA III used by high-end desktop PC’s). I can see a lot of demand for RAID 5 storage arrays connected back to the this new Mac Pro storing several tens of terabytes of data.
They really could have used a few PCI slots for things like 10GB or 40GB ethernet, and whatever comes down the pike. I wish they still kept the “big box” format around in conjunction with the new “tube” design....