For me, Starbound was 3 decent-but-not-great games that were in no way cohesive.
Exploration, building, and RPG/quests
Exploration was a big selling point for the game. I think we were all still enamored with the randomly generated planets idea from Minecraft, but it was fairly engaging for far longer than it had any right to be. Interesting POIs, nearly endless supplies of weapons and gear. It fell short when it came to enemies, but apart from that it was impressive.
Building was really interesting too. It built on Terarria in a way that allowed you to make anything from an underground space bunker to a castle. Unfortunately, there was so many building items that you would end up just using the pixel printer if that's what you wanted to do, and would kind of ignore the exploration aspect. It also really didn't make sense to build anything when you could customize your ship.
And then the quests. I was so disappointed with the story and quest system. Every story based area existed in some kind of pocket dimension that was totally separate from the universe you're jumping around, so it feels like the open world is intrinsically limited in how interesting it can be, since you know anything really important has to be immediately teleported to.
It was difficult to focus on all of these aspects, since they were deep enough to be interesting, but couldn't stand on their own, so you found yourself only dipping your toes into each area and leaving fairly unsatisfied.