KSP [official site] will continue to be a work-in-progress – the developers say as much – so what does leaving beta actually mean in this situation?
Well, it means that the version contained in the next update (1.0) fulfills the goals laid out in the original design document. In that sense, Kerbal Space Program will be a complete game and will graduate from the Early Access program on Steam.
At that point the developers' agenda is going to switch to… more Kerbal. As they put it:
"We see no better way to follow up on reaching our initial goals than to continue development on Kerbal Space Program, beyond Beta, past our original plans. This means that after 1.0 is out, we will continue on with free updates 1.1, 1.2, and so on."
There are a handful of details listed on the blog regarding what the 1.0 update is set to include so here's what stood out to me:
Game Over – you can actually lose your job at the Space Centre if you're careless with your funds or your reputation. As someone who has routinely smashed Kerbals into the ground this fills me with a sense of dread and career mortality.
Engineer's Report – a panel which warns you of acts of stupidity in your craft design like leaving the engine out.
Deep Space and Planetary Refueling – a system and parts which let you process matter from things such as asteroids in order to make helpful substances. Fuel and oxidiser are the examples given.
Female Kerbals – Female Kerbals are now part of the space program. Well. Until I send them on another one of my famous probably-almost-definitely-non-lethal missions, anyway.