It seems only a few months ago when Google released Android 4.3. Actually, it was only a few months ago; such is the pace of change and improvement within Android. At the end of October, Google announced the new Nexus handset, the Nexus 5, plus Android 4.4.
Just as Android 4.3 looked like a small improvement to 4.2, so at first 4.4 feels like a very minor revision to 4.3. The status icons are white rather than blue, the shortcuts have changed but it looks and feels very similar.
There are some important changes under the skin and the first to discuss is battery life. Now, long time users will understand that every software update does a few things, including fixes bugs and improves battery life. On my own devices, it's too soon to be able to quantify the difference and in any event, I've not moved from a day to three, but there is a noticeable improvement.
The second is in performance, where Google have optimised 4.4 to require less memory and so run better on lesser handsets. It's difficult for me to quantify any changes in smoothness and performance on my Nexus 4 because I've never found this wanting, but on my 2012 Nexus 7 there's a definite improvement across the board. It isn't as responsive as my Nexus 4 running 4.3, but it's a big step in the right direction.
Android 4.4 now includes an experimental feature called ART, which you can read up about here.
There have also been more changes to Bluetooth under the skin including better compatability with hands free car kits and similar, although I've not noticed any difference after the significant changes Google implemented in 4.3.
Google have revised and simplified how Android detects location, making it clearer how devices determine where they are. We now have three modes: high accuracy, battery saving and device only. High accuracy uses GPS, WiFi and mobile network masts for location sensing whereas battery saving disabled GPS. Device only uses only the GPS and does not use mobile or WiFi networks. Furthermore, the location settings now shows what applications have accessed your location and how much battery the application has used accessing location.
From a productivity perspective, the biggest improvement I've seen is how Google have better integrated Google Drive and Quickoffice into the operating system. Whereas before, Drive felt like an add-on component to Android and wasn't especially well blended into Quickoffice, now it's much tighter. Quickoffice is now part of the operating system and there have already been updates available. These changes, combined with the improved battery life and system responsiveness, polishes Android still further.
From what I've seen and read about Android, however, the most important changes to Android in the latest version will only be obvious when it starts to arrive on low and mid-range handsets. It's no surprise that Android is fluid and fast on a 1.5 GHz quad core processor backed up with 2 GB of RAM, but it'll be interesting to see how well it runs on something with a 1 GHz single core processor and 512 MB of RAM.