From the official announcement:
What's new:
Upgraded to Android 4.4.4
Updated to new versions of Google apps
Added Clear Image
Added new (colder) calibration for the screen
Proximity sensor prevents activation of off-screen gestures while in your pocket
Capacitive touch keys supported in the recovery mode
Increased capacitive button illumination
Improved overall stability
Fixed issues with vibration being disabled during Quiet Hours
Fixed lockscreen not updating the track information during music playback
Fixed microphone volume for all formats in the Voice Recorder app
Fixed “OK, Google” not triggering voice search in Google Now
Fixed Viber calls not working
Fixed battery percent not updating in custom lock screens
Fixed PicasaSync
Fixed Camera “O” gesture activating lockscreen instead of triggering camera
Fixed wallpapers not applying in full screen, taking the wrong size when cropping, etc
From Android Police:
Update: In addition to the above, we've noticed the following changes:
Speaker volume while on a call is higher - this is a very welcome change, as the speaker used to be practically unusable unless you're in a totally silent room.
Mic sensitivity when using voice commands is much improved - the One can finally hear what you're saying at an arm's length or more rather than just about an inch or two. If you use Google's Search app and interact with it using voice, the differences you will see are going to be night and day.
The mobile data symbol in the notification area now shows LTE instead of 4G when using an LTE network.
As part of Android 4.4.3/4.4.4, you will see a revamped dialer.
The model number is now being reported as A0001 for some reason.