Perhaps the title of this release should be "too much stuff to list." The full release notes are on the Arcen forums.
One of the big groups of changes here are the Hybrid Hives. Their functionality has been tuned and extended quite a lot in general, and some new interface elements have been added to the galaxy map for working with detected hives, etc. They have also been nerfed rather heavily, as they were brutally destroying most experienced players within the first two hours of the game. These are meant to be an exceptional challenge, but not quite to that degree. There is a new "Advanced Hybrids" AI Plot option available for CoN owners that adds on the nastiest parts of the hybrid logic, and keeps the challenge at a very high level. This lets players adjust just how steep a challenge they would like.
Also in this release are a wide range of bugfixes, and one rather notable performance improvement. Of course, all bugfixes and core improvements of this sort are free for all players -- they are never tied to any expansion or anything like that (we think charging people for bugfixes would be pretty unethical to say the least).
There's also six new command stations for players to choose from in two new command station lines. Previously there were only four command stations total, split across two lines total. This more than doubles the number of strategic options that players have for colonizing planets. Best of all, these six new command stations are for all players of the game, not just CoN owners. Though there will be some new CoN-specific command stations coming soon...
There are also four new warheads available for all players: the old Lightning Warhead is now Lightning Warhead Mark II, and new Mark I and III variants have been added. This adds a bit more flexibility into lightning-warhead-related strategies, which should be quite welcome with many of our expert players (who use them extensively). Similarly, there are now Mark II and III variants of the EMP warhead. Often players were having to "stack" the effects of the EMP warheads to really make use of them, and these higher-level EMPs provide an alternative to that. Though clever players can cause the lowest overall AI Progress increase simply by sticking with the lowest-tier EMP and combining that with transports and/or starships.
The last major addition is to the mercenary space dock. First of all, it's been visually revamped and now costs 0 energy to run. But players also now get a starship constructor, warhead silo, and mercenary space dock right at the start of each game; this makes for faster starts for expert players, and helps newer players realize what is available. In the past, there were only fighter, bomber, and frigate mercenary variants, but that was somewhat limiting and basically only something people used in order to enlarge their fleets after they had already hit ship cap. Now, there are also Parasite and EtherJet Tractor mercenaries for everyone, Beam Frigate mercenaries for Zenith Remnant owners, and Neinzul Enclave Starship Mark I mercenaries for CoN owners. All four of these new mercenaries come with the usual 10x-cost-but-no-ship-cap-and-no-unlocks-required common to the existing mercenaries, which makes them much more interesting for during general gameplay. Players are much more likely to go for some of these specialized mercenaries, despite their exorbitant metal/crystal cost, to gain access to functionality they might not otherwise have in a given game, not just to pad out their fleets.
All in all? This is one huge prerelease, and it substantially alters the number of strategic options available to players. We expect there will be some rebalancing needed for a lot of the new stuff, but these initial versions look quite promising already. Enjoy!