Primarily playing single player, the AI is a very important element for me. It seems, however, to have taken a large step backwards in the Rebellion Beta, or as if about half the AI code was forgotten(?). The AI is buggy and often unresponsive, getting stuck in stalemates in every game I have played so far.
In every game so far, once all the planets have been colonized, the AI seems to just shut down. I have tested this from Easy to Unfair and 2v2 to 6 player FFAs. Even in a 2v2 with all the AIs on Unfair/Aggressive, they end up sitting with their entire fleet on one planet, building a starbase there, and waiting. No AI is willing to attack their neighbor so the game ends up as a boring stalemate with me being the only one doing anything. This never happened in the other versions of the game.
Once the game enters this stalemate state, the AI doesn't do anything to protect its other planets, and retreating forces stationed on them. Once, in a 3v3, one of the AIs did not stalemate like this, because it only expanded to four planets, so it stayed aggressive and attacked my ally. My ally sat there, his whole fleet (titan included) on an asteroid across the solar system doing nothing but spamming me with "It's not looking good for X" messages while half his empire was conquered by blue.
AI Capital Ships
From what I've seen so far, the AIs almost never use their capital ships for fighting. They send their frigates and cruisers to colonize a new planet while the capital ships patrol the back of their empire. This makes the AI fleets pretty easy to beat with so much of their resources in capital ships they aren't using.
It seems now that the AI doesn't care at all for their planets if I arrive with a larger fleet than them, especially if I bring a titan. The AI fleets retreat as soon as I enter the gravity well, leaving the planet wide open for me to bomb and take.
These problems, for a single player person like me, are the biggest things that need to be fixed before release.