Narrowband Sensors
Orbital Habitats (these are a cool feature)
Ordinance Manager in the ship menu (very handy)
Tractor beam bug fixed (sometimes would have problems moving around other objects that weren't shipyards)
(not sure?) Seems fighters have a proportionally smaller number of crew now compared to 5.14 but im not sure if this is a bug
You can pick which precursors you wish to have present in your game out of the traditional prescursors, Star swarms and invaders.
I think steve said there were some performance improvements as well (have a read of the 5.14 bug report steve has replied to peoples reported bugs he's fixed)