My observations; all from Tough games, mostly small and medium galaxies with a few larges and tinies.
The Terrans are good expanders, but don't militarize much. Or at all.
The Drengin are a bit slower at expanding than in DA, but just as aggressive. I thought they were a threat then, I think they're a threat now.
The Altarians aren't THAT fast at expanding early on, but they get extreme colonization techs pretty quickly. They get planetary invasion, but seem as reluctant as the Terrans to build an actual military.
The Arceans are slow expanders due to their -1 speed and their tech tree, but they do build a military fairly early on. I haven't seen them become one of the top races.
The Torians are incredible. Real fast at grabbing planets, with Super Breeder giving them a powerful early economy. They build vast amounts of tiny ships early on. They even get Aquatic colonization early due to their tech tree, usually giving them even more planets. Usually VERY powerful.
The Yor have potential sometimes, and other times they just flop. I've seen them never getting any planets outside their home system, and I've seen them grabbing the most planets of all the races. They do militarize, although not to the extent some other races do. And their super ability is bloody annoying.
The Korx don't grab a lot of planets, and like the Altarians, they build transports but no armed ships. And they often get much of their empire culture flipped by the more expansionist races.
The Drath aren't too fast at grabbing planets, and not too fast at militarizing either - but unlike some of the other races, I've actually seen them build armed ships. They're dangerous because of their super ability, but once you can get to them, they're fairly easy to wipe out.
The Thalans are even faster than the Torians at grabbing planets, but don't like building an early military. In fact, in my last game, I let them live because I thought they never built one. About five years in, they researched weapons, and already had large hulls. I don't think they're very dangerous - they're great for an early conquest though.
The Iconians handle themselves a lot better than in DA, poor bonuses aside. They're decent at planet grabbing (their super ability probably helps), and they do build a military. Still, they're not threatening, but a pretty balanced opponent.
The Korath are just as threatening as they were in DA. They store large amounts of spore ships (although they don't put as many engines on them as they could) and have the potential to build incredible militaries, especially when they get Ion Beam techs. Yet they're not as aggressive as they could be early on. Still, they scare me enough to leave them out of the game much of the time.
The Krynn are fairly good at expanding, and fairly good at militarizing too, although they seem to stop at small hulls with the basic weapon that researching space weapons give them. It's annoying to share a border with them because of their influence bonuses. I don't think they're as powerful as they were in DA, but a fairly balanced opponent still.
Whew, that's a wall of text.