There were a few things.
After the Damage 2.0 rewamp they have changed the meele system as well. Now you can shift your focus to firearms or meele weapons.
Two or three new warframes were added and the UI was changed (lokks a bit like in Dead Space now).
The new UI brought a personal space ship as well and new tutorial missions.
Most of the new contend has involved a lot of grind (i have need ages for an kubrow egg, and the kubrow itself works only as money sink), Archwing on the other hand was quite easy (i guess the flight suit will be important later on).
And they have added a few more game modes, some conquest thingy, excavation (thats a variation of the survive mode), some missions with void portals, escort missions, hostage rescue has become a bit harder, archwing missions and group PvP.
Oh and a few more enemy types.
Overall it is still a lot of grind for better modules and blueprints or you take the short cut via real cash.