Bearing in mind that while stable features should be merged first, we can't skip out on any features, or we will break existing mods, which is one of the worst possible things we could do.
It's pretty hard to define stable, and GAE has a lot of features... Subfactions are pretty stable, but the AI cannot use them at all (game breaker!). Emanations *seem* stable, but have only been used to some degree in these recent versions. Save game is stable in the latest stable release, but holds the reward for being the most often broken feature. The pathfinder is stable, but if I recall, has given problems when trying to merge in the past (?). MG particles were a bit buggy last I saw, though I think that was fixed in the RC.
Of course, I could be wrong about any of these. I haven't gotten the time to test the RC (busy busy busy), and as I said, it's hard to define stable (in my case, some are just "works [flawlessly] for me").