It doesn't look big enough to store any internal 3.5" drives. The 2006-2012 Mac Pro had internal slots for 4 drives. That is the main thing I have been looking forward to in a new Mac Pro and is the main reason I haven't upgraded past my 2010 Mac Pro. We are just supposed to have all our storage in external drives from now on? I don't want all those wires.
If you're dishing out > $10K for what's basically a supercomputer for the masses, will you really want anything that fits into 3.5" bays? I wouldn't dare throw HDDs into this machine. I guess some people still need optical drives, but the external ones seem to do the job just fine.