First of all it's hard science-fiction, and second of all, it has an ambitious feature list that boasts an advanced economy, space battles, seamless planetary landings, and "lots of engineering." It's also got a nifty ship system wherein players will need to manage a generator and wattage to power various onboard systems.
Finally, there's a fully functioning computer within the computer game. Notch says that the "emulated 16-bit CPU can be used to control your entire ship, or just to play games [...] while waiting for a large mining operation to finish." The only question left in our minds (other than when can we play it) is whether or not it's an MMO. The website hints at single- and multiplayer functionality "via the multiverse," and it also mentions a monthly fee.