Defect numbers from my list I would consider showstoppers:
605, 609, 616
For an overall review, I have played this for a couple hours now using cheat codes because I don't want to invest 20 hours into a minor release of a beta...
My initial impression was, wow this is really slow. And I realized, I have the profiler set to insane. I didn't notice anything cool about setting it to insane, and going on to the big battles with it on makes everything feel sluggish. I am running a i7-930, 8 gb RAM, but when I checked my performance meter, it doesn't look like the game is able to access the other cores. The memory usage on a large fight is about 1 GB, for 2000 ships of mine and five golems invading a planet with less than a couple hundred enemies.
Briefly, the engine doesn't take advantage of a beefy machine. I think it would perform better on a core two duo with a higher clock speed than a nehalem. My clock speed is about 3 GHz with turbo boost on (automatic overclocking). I don't think the last one did either, but it's a little more noticeable with the performance requirements of unity.
From interface standpoint, the explosions are pretty, and the force fields look awesome. I enjoy the new menus, the opening screen, but, I have one other reaction, and that is things felt a little "thick." If you look on the ship sidebar of the main gameplay screens, each of the borders are now glowing halo instead of the thinner lines of the last engine. Planetary names now have a circular halo around them that makes them even bigger than the font, which is also really big. It is so big, in fact, that it covers the units that are designed to guard wormholes. I opened a defect on this, but I was told it is an engine limitation. The message advising that "you lost" is incredibly huge, large and flashing, fitting the overall theme of everything getting thick graphically.
I am a little sad to see that the game is moving away from large space battles. We now have guardians, which I think an open some AI game mechanics worth seeing, but I did like having a giant space battles. I'm going to keep my old version of this game probably just for that reason alone, because everything felt more epic in scale. This game was never about high-tech graphics, and it still isn't, but I feel like we are losing a lot of the abilities of the older engine in favor of multiplatform capability. I understand that's the way we are going, but I haven't seen anything so far that you couldn't do in the last engine, except put it on a Macintosh. Your existing players, I'm not sure that unity offers anything for them that you couldn't do with the old engine. I haven't seen anything that makes me think, "wow, I'm glad unity is here, because now I can do x,y and z." That's all I'll say on that, I know there's a lot of excitement, so on with that... I think what I'm most excited about, is that we are hopefully going to get back to the gameplay mechanics themselves, new AI, new actual ways of playing, and all of this engine and UI stuff will just be the way we interact and no longer the focus like it is right now.
My other feedback exists in all of the entries in the bug tracker, will be awaiting new releases. When you talk about showstoppers, what you need most of all are the basic mechanics working, such as being able to move ships around, travel through wormholes, and able to control battles. I haven't seen anything really game breaking about controlling battles, but the first two are not ready yet, and that's something that would stop a release.