Only ~350mb or so. For something that needs as much work as the Xbox One, I'm doubtful there's much in here worth getting excited about... We'll see.

Update from Polygon:

According to Xbox support, the latest version of the Xbox One system software adds the following new features:

Update to My Game & Apps to allow sorting of games and application lists, and separate queue lists for games, applications and installs
Ability to delete game and application save data
Improved installation and DLC management
Addition of the controller battery power indicator
Improved UI for accessing friends, achievements, messages, and party chat
Game DVR app to be included in the Xbox OneGuide as an app channel
Update to the boot progress indicators for system update
Improve consistency of UX for update & install progress
Separate game, application, and install queue lists
Addition of USB keyboard support

The update also includes a handful of stability and performance-related changes:

Microsoft also confirmed earlier this week that the update makes some slight changes to the Xbox One's thumbstick sensitivity.

Update 2 from Major Nelson:

Tip: Once you get the update, you can press the Xbox button to see the battery indicator on screen in the lower right next to the clock.

Also here are some notes about what is in this release:

Xbox One Update Live This Morning