As written in the previous article, SteamOS has its own graphics compositor while SteamOS is based on Debian Linux. In digging through the steamos-compositor code, it's a modified version of xcompmgr.
Xcompmgr is a simple composite manager for X11 written a long time ago by Keith Packard and duesn't see too much activitity these days. The xcompmgr package is still popular though among lightweight Linux users. In looking through the code, steamos-compositor is much-changed compared to upstream xcompmgr. The diff between steamos-compositor and upstream xcompmgr is over a 4,200 line patch.
SteamOS is using the Linux 3.10 kernel, but it's not a vanilla kernel. There's many patches added onto the Linux 3.10 kernel for SteamOS, particularly taken from the real-time patch-set and other changes. As pointed out by a Phoronix reader, "SteamOS appears to be using linux 3.10 - PREEMPT_RT_FULL (unsurprisingly) with a heavy amount of patching (282 patches for -rt in the 'all' architectures/folder, alone). The kernel is also using aufs and they seem to be sitting on some bug fixes for upstream on top of that. Some of the -rt related hacks they are using i have seen (in one case, i am using the same patch for ntrig)... It looks like they have gone to a lot of effort getting the kernel just right for their needs."
SteamOS is using SysVinit as its init system (no systemd or Upstart) and is just using an X.Org Server 1.12.4 without Mir or Wayland. The Catalyst and Mesa drivers are present on the system, but SteamOS Beta 1 is being advertised as NVIDIA-only.
SteamOS benchmarks are forthcoming on Phoronix! Follow @MichaelLarabel for real-time updates.
UPDATE: New details in this article! There's also now the first SteamOS benchmarks.