Starbound is a 2D, procedurally generated world where the player will explore in order to obtain new weapons, armour, and miscellaneous items. It has a heavy focus on crafting and exploration, and you must do both extensively in order to progress further into the game.
Pro Incredibly deep gameplay
The exploration, crafting and combat are all both very polished and deep in terms of what you can do, especially compared to other games in the genre. This makes the sandbox very fun to play in as you're never really stuck doing the same thing over and over.
Pro Delightfully challenging
Starbound is unlike most games in this genre, in that from the very beginning it has a decent level of difficulty to it, which for a lot of people is a very nice change. The very first enemies you encounter can get the better of you, and materials will take awhile to harvest. Don't expect rapid advancement unless you get very lucky.
Pro The game gets even better with multiplayer
Play with your friends! It makes base-building and exploration much more fun, and you'll also advance to the more impressive craftable items more quickly. Of course it will take a decent level of coordination in order to make the best use of having multiple people work on a single aspect, but that's part of the fun.
Pro You can unlock new sectors of space to explore by facing several different bosses
By facing the Bosses of Starbound and defeating them, you open up the universe even more. This essentially means that by killing a boss, you increase the diversity of the materials you can obtain, which also increases the customization you can do of your character through new and improved gear.