Developer highlights: http://developer.android.com/about/versions/kitkat.html
Android Developers Blog: http://android-developers.blogspot.com/2013/10/android-44-kitkat-and-updated-developer.html
Videos (an overview one plus lots of others that dive deeper into specific features): https://www.youtube.com/watch?v=sONcojECWXs&list=PLWz5rJ2EKKc-2quE-o0enpILZF3nBZg_K&index=1
I'll specifically call out the video on Transitions, which is something I recorded last week after finally finishing the feature. I hope that the API is another step toward making animations easier and more automatic in Android applications.