1 - Star Control 2. I mean, 1 was great when it was new, but it was kind of primitive, and difficult to play after 2 came out. And 3 was garbage.
Any Star Control game built to feel like a sequel to 2 will already be on the right track. And so far, that describes this one; everything I've seen makes me think it was built by fans of SC2.
2 - If what we're hearing in fleet battles is any indication, you guys already have the music down; what we've seen of the game so far feels like Star Control, and the music is a huge part of that.
3 - The aliens. Their unique cultures, their unique looks. Talking to them, interacting with them. It's like Star Trek or Babylon 5. Or Farscape.
4 - Exploration. The rather gigantic universe. The unique planets. I'm not sure whether I went to every planet in SC2, but I went to a lot; the fact that I probably haven't been to them all says a lot about a game and its replay value. I know a lot of games these days have the whole procedurally-generated-universe thing going on, like Spore, Elite:Dangerous, No Man's Sky, etc...I don't know what the plan is for this game, but if it boasts that whole billions-of-unique-worlds thing--and does it well--then this game's gonna be a classic.
And no. damn. star-lanes!!! The Master of Orion reboot made the rather unfortunate mistake of making planets connected by specific warp corridors (or whatever they were called; wormholes, maybe), so even worlds right next to each other might take several jumps for the trip. SC2 didn't do that, and this game better not either.
5 - The story. SC2's story was pretty epic, with a huge backstory that led to present-day legends that led the player down the path to the game's conclusion. Most of the alien races were involved on each side of the conflict, and the player got to help it all unfold. It was very similar to Babylon 5 in that way, and I hope this game has something similar. (It'd also be great if there was no single ending, that the player could affect events such that one side might win, but starting over from the beginning, another side might. Or something like that. Make the game even more replayable.)