IMHO error rates become a concern at these densities.
Backup is another concern - hose a 3TB drive and there goes everything! (or if it's lost, stolen, etc.)
At any rate, of course the better capacity and affordability is welcome. Applications just have to scale up to take advantage of these mainframe-like capacities.
SAS stands for "Serial attached SCSI" - not a typical desktop interface, but I'm sure SATA versions will follow.