Compared to Terraria... It looks near identical with a few changes, I'd hate to purchase a game that is merely a clone of another game, unless it significantly improves on a lot of the aspects of the other game.

In short, is Starbound a huge improvement or is it about the same as Terraria?

I know people who've purchased it and they love it. They played terraria also and as Illauna mentioned it is the same basic game. If you liked terraria you'll probably like star bound. But again to agree with Illuana if your looking for some ground breaking changes than it's not quite to that point yet. You do get a ship and you can fly around the galaxy (not literally fly it but navigate, click and warp type). Planets have different threat levels or something like that. to start you want to find a planet with a threat level of 1 and then you can move to different planets as you acquire more fuel and supplies.

I myself am on the fence on buying it, i've always been more of a minecraft person, although i have terraria and i liked it i still preferred minecraft. But the idea of how they added the planets thing sounds attractive. Well that and how you can make so much more in terraria (and star bound) than you can in minecraft.