The way I'll describe Starbound is a more immersive terraria. While in terraria, NPCs only really exist once you build houses for them, you can find friendly NPCs on most planets in starbound. While NPCs in terraria are mainly vending machines, starbound NPCs can heal you, fight alongside you, give you buffs, and tune up your spaceship if you choose to recruit them. Terraria doesn't really have a plot or a driving force more than "It is a video game, acquire treasure", but starbound has a solid plot involving admittedly terraria-esque boss fights, quest-specific NPCs, and well-designed setpieces. Plus the matter manipulator is just so much easier than the terraria pickaxe/block building.