Biggest feature is the Sync Gateway Accelerator component, which makes SG a lot more horizontally scalable; that should be a big deal for larger deployments.
CBL changes are mostly bug fixes. On iOS the biggest fix is better error handling when filesystem access is lost because the device is locked. (Apparently iOS 10 got a lot more aggressive about locking the filesystem, because we started getting bug reports about this shortly after its release.)
You can of course get all the detail you want by looking at the commit logs on Github
All I can say about availability is "real soon"...