A summary of the differences, for the curious:
Valve has removed a pointless second copy of the package manifest. Good, we only ever kept it because Valve did
Valve doesn't include the package contents manifests. These aren't useful for 99% of people anyway
Valve doesn't include all the firmware ever. In Ye Olde SteamOSe, we added every bit of device firmware possibly used by end users. Valve does not include all of them (they already included a couple, e.g. for Realtek networking)
Valve has rolled in some recent Debian security updates, e.g. to apt-get and curl
Valve includes all the 32-bit stuff there is. In Ye Olde SteamOSe, we strip out most 32-bit packages which are not used. Valve includes them all, so has a bigger ISO
Valve includes the 32-bit version of the installer - no 32-bit kernel though, so not usable on 32-bit-only computers.
Valve does not include support for LVM or mdraid
Valve includes an installer which runs from Windows. I don't know what functionality it allows or prevents compared to booting the installer directly
Valve doesn't contain Ye Olde SteamOSe's audio hacks - it still only works properly on Brix or Prototype
Valve has killed the old "expert" mode - Ye Olde SteamOSe's "Power User" mode is included instead, relabelled as Expert. YOS still includes both Power User and the old-style Expert modes
Valve doesn't allow NTFS resizing (
https://github.com/ValveSoftware/SteamOS/issues/88 )
Valve doesn't include VirtualBox guest additions. But no big loss, given VirtualBox is shit.
The headline features pulled in by Valve are:
Non-EFI support
DVD install support
Partitioner in Power User/Expert mode
Dual boot in Power User/Expert mode
Supports recovery etc if you use a custom layout as long as it's still Valve-ish enough