Headed over to XDA to install the new 4.2.2 Google ROM for the GS4 and noticed there was a leaked 4.3 ROM that had been ported to the i9505, fired it up. Apparently most of the changes are under the hood, after watching Googles presentation they talked a lot about optimizations for the GPU in drawing the objects on the screen. Something about 2.5x more efficient. Most noticably the new Camera UI is here. Android Central mentioned stuff about new Bluetooth Stacks buried in there.
(Shrank down the screenshots hugely because they'd be huge otherwise)
Anyone who knows more about it want me to do some digging?