Most of the new content comes from the changes that Barking Dog studios made to the game's engine and how they chose to highlight other existing parts that were simply underused in the original. The biggest change is how the game manages fleet size. No longer is there a cap on the number of a type of vessel you can build. This also eliminates a problem that Homeworld had, as you cannot salvage a vessel that would put you over your support limit. This also forces you to think about fleet composition and adjust it in the middle of a mission, retiring unneeded ships to make rooms for the ones you'll need to finish.
Research also plays a larger role, with research broken up into different areas.
Much of said research is for enhancing existing ships, requiring them to dock or stand down for a short time to upgrade. The story line is darker and stronger than the original, with a real sense of desperation. Along those lines, the mission structure can be rather grueling, but in a good way. Sometimes it forces you to abandon ships and, in a few cases, literally turn tail and run. If you like games that advance the story in play, you'll not be disappointed here.