Docking
Crew diversity
Boarding
Hinges + Sliders
Resources + Trading + Salvaging
Cloaking Device
Blink Drive
Fighters + Carriers
Ramming
Stations
I'm pretty much looking forward to all the new features. But being able to dock ships, transfer resources, and do boarding actions, are my favourite.
Being able to combine ships and have them transform into a massive robot is a close second.