It also looks like the current form factor also limits internal storage to a single drive - i.e. it's a workstation that does not provide for an internal RAID configuration. It also does not provide any way to back data up without using a network connection or external device.
None of these may be deal breakers, of course, but the trend is definitely away from upgradability. Then again, a shared heat sink and limited air volume combined with a 450w power supply and a single fan design probably give Apple business reasons to move in that direction.