I've mentioned previously that the OS reservation could include gaming functionality. However, things like progressive download shouldn't need oodles of RAM. Nor streaming. XB1's justification came from large datasets for Kinect. PS4 doesn't have that, so 1 GB should be ample for multiasking lots of services and features.
This conversation has basically run its course now though. There are those who say, "I can't believe it," and those that say, "it's obvious, these consoles have to do everything a PC and tablet does," and no-one can provide any more details than that, so I guess I'll just step out here and leave everyone to call each other stupid for not seeing the mind-numbingly obvious.
Maybe when Sony show the system in action it'll be apparent what is consuming all that capacity.