This update is focused mainly on optimization, improvement and graphical upgrades, with a few key gameplay features thrown in. The first worthy mention is the new Astronaut Complex facility, allowing the player to selectively recruit astronauts, a feature that will be especially important in the future "Career" mode. Along the same lines, a new Crew Management system gives us the option to pick and choose which astronauts will crew our planes and rockets. One last, and to my mind, most crucial new gameplay upgrade: the SAS/ASAS system has been entirely upgraded to be more responsive and less "twitchy," and to allow for smooth control of vehicles while activated. It's a game-changer.
Also improved are the Vehicle Assembly Building and Spaceplane Hangar (a screenshot of the new VAB leads this article, and more can be seen in Scott Manley's preview video). The level of detail is impressive and has been upgraded across the whole Kerbal launch complex. In fact, the entire planet of Kerbin - along with the Mun, Eve, Duna, and Moho - have been upgraded to provide more varied and interesting terrain.
Finally, Squad has released an excellent video with this release, this time "highlighting" the new astronaut selection system.
After being prominently (and repeatedly) showcased in the recent Steam Summer Sale, we expect that this will be the first Kerbal Space Program update for many new players, including some of our readership. Enjoy!