Details on Android 4.4 (KitKat) are available, and it'd be hard to find a more thorough review than yesterday's from Ron Amadeo at Ars Technica. According to Amadeo, there are a lot of big changes to be aware of:
We believe KitKat is the biggest Android release since Ice Cream Sandwich. Google has touched nearly every part of the OS in some way, so there's a lot to cover.
Most of the review is a direct comparison between KitKat and Android's previous generation, Jelly Bean, with changes explained and side-by-side pictures for everything that's noticeable. Unfortunately, some really integral tests are just not possible yet, such as memory-usage comparisons for low-end devices, because KitKat is currently only running on the Nexus 5.
Otherwise, though, just about every new change is covered, and while some are primarily of consumer interest, many are need-to-know for developers as well. New system and status bar interaction with full screen apps, for example, is notable.
Ars Technica has a lot of good to say about KitKat, but they suggest that there is still work to be done, and Android still lags behind competitors in some areas. Check out Ars Technica's full review for all the details.