Android 4.4 (KitKat)

Android 4.4 (KitKat) introduced a number of new features: integrated messaging app (Hangouts), wireless printing, improved immersive full screen mode, new Dialer app, improved stock Email app, NFC update which allows apps using NFC (e.g. Wallet) to function without the access to secure element on the device, and many more. Android 4.4 has also been optimized for more efficient memory usage, which should improve overall performance.
Below are some links that will be useful for consumers:

• What’s new in Android 4.4 - official Android site
• Complete Android 4.4 FAQ  by JR Raphael

• High level release notes
• More detailed release notes
• API 19 differences report
• Potential Android 4.4 compatibility issues by Mark Murphy