I hope people realize that if it was being rendered on remote servers, over the internet, the moment you lost your internet connection, you wouldn't see anything new and would be stuck on the same, degraded, or blank, screen?
I also hope that people realize that this is not the case...
There is some contention on that theory as every time I log in since the latest patches if I have pending awards from crafting quests or even a guild MOTD if I log in at the Fleet I have an entirely black screen until I clear these UI elements.
Then the world graphics resolve in, and I do mean resolve in, I can literally watch the step up process in motion. And I'm on incredibly high end hardware here so its not an issue of my computer being unable to handle the load.
I don't think they're streaming 100% of the game but there is evidence that supports that remote asset streaming was in use as early as the 2009/2010 beta tests and I still believe that some assets are being streamed, though attempting to stream player assets of this magnitude would be phenomenally stupid.