The full release notes are on the Arcen forums. This beta has a mix of new stuff for the base game, as well as specific to the Children of Neinzul micro-expansion.
The biggest new addition for the CoN expansion are the new Neinzul Clusters and Neinzul Nests, and their related new ships. There are a good dozen specific structures between those two categories, with varying mark levels and personalities. The Clusters let off bursts of Neinzul ships whenever they are antagonized or killed (think of stepping on a spider's egg sac -- yick), and if they survive the antagonization they recharge their cache of Neinzul ships and will later send out more the next time they are antagonized. The Nests are similar, but are also angered by the humans controlling their planet or an adjacent planet, or
killing AI units on their planet or an adjacent planet. There are some other various differences between the two, as well, that the release notes and in-game tooltips cover in more detail.
Going along with the new Nests and Clusters are the new Neinzul Bombers and Neinzul Viral Swarmers, which are small AI-only fleet ships which get released from specific types of clusters. The first two AI Types for CoN have also now been added in the form of the Neinzul Viral Enthusiast (easier) and the Neinzul Nester (harder). The AI also now uses Neinzul Regeneration Chambers.
In terms of what has been added to the base game, it basically boils down to ion cannons and starships. Since 1.0 and before, there have only ever been two marks of ion cannon: mark I and mark II. Well, the time has come for marks III through V! Those very advanced weapons are now in the hands of the AI, but only on the higher difficulties (see release notes), and only fairly rarely in any given galaxy. It's a good idea to use starships or very high-level ships to take these out, probably with the aid of transports to dump your attackers right next to the cannons.
In terms of what has changed for starships, the dreadnoughts have now had a significant health boost and a significant speed boost to their energy bombs (which also applies for a few other ship types, such as Zenith Bombard Cannons, actually). The flagship, Zenith starship, and spire starships have also had their health doubled, partly in reaction to their recent knowledge cost increase, and partly due to the new survivability of the dreadnoughts. Lastly, there are now three new marks of scout starship (marks II through IV, to add to the existing mark I). These starships provide combat support or excellent scouting capabilities, but at a pretty steep cost. The mark IV scout starships are even more useful than the venerable mark IV scout from the advanced factories, but the knowledge cost to get the highest-tier scout starships is three times as much.
Another notable change is that the AI will now sometimes make use of mobile repair stations and space tugs, or of clusters of spider turrets.
More to come soon!