Lots of stuff, though if you can wait a little bit longer I would wait a month or so for the next update, 1.5 Banks, which will revamp and improve a lot of features relating to ethics, factions, and pops.
I will add a few things to the list I suppose.
They overhauled ship building, added humanoid phenotypes, and the habitable planets are now divided into three climate types, wet (continental, tropical, ocean) dry (desert, arid, savannah) and cold (tundra, alpine, artic) planets that are the same type as your homeworld are 80%, same climate type is 60% everything else is 20%.
The DLC pack is called Leviathans, and along with what lordmagus said, they added Enclaves, special stations with traders, artist, and scientist who can offer their services to you, and Guardians, ancient creatures (each is unique) that guard some sort of treasure or secret in their own system.