Make with the details.
The major changes made in SteamOS are:
Backported eglibc 2.17 from Debian testing
Added various third-party drivers and updated graphics stack (Intel and AMD graphics support still being worked on)
Updated kernel tracking the 3.10 longterm branch (currently 3.10.11)
Custom graphics compositor designed to provide a seamless transition between Steam, its games and the SteamOS system overlay
Configured to auto-update from the Valve SteamOS repositories