Please include the system information requested at https://github.com/ValveSoftware/steam-for-linux#reporting-issues.
Do you have your graphics drivers (either mesa or fglrx/nvidia) installed correctly, in their i386 variant?
It appears that your GL settings are improperly configured.
Search on Google for Xlib: extension "GLX" missing on display ":0.0":
https://www.google.com/search?q=Xlib:+extension+"GLX"+missing+on+display+":0.0"
Closing the bug - please reopen if it is a bug in Steam.
Nothing extracted yet.
When I run Steam from the command line, I get this.
michael@Michael-Lenovo-IdeaPad-Y570:~$ steam
Running Steam on ubuntu 12.04 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(1360439662_client)
Xlib: extension "GLX" missing on display ":0.0".
Looks like steam didn't shutdown cleanly, scheduling immediate update check
Installing breakpad exception handler for appid(steam)/version(1360439662_client)
Installing breakpad exception handler for appid(steam)/version(1360439662_client)
unlinked 1 orphaned pipes
removing stale semaphore last operated on by process 27508 with name 0eBlobRegistryMutex_27E403C6ABC5F39E6B01561C6E60398E
removing stale semaphore last operated on by process 27508 with name 0eBlobRegistrySignal_27E403C6ABC5F39E6B01561C6E60398E
removing stale semaphore last operated on by process 27508 with name 0emSteamEngineInstance
removing stale semaphore last operated on by process 27508 with name 0eSteamEngineLock
Xlib: extension "GLX" missing on display ":0.0".
OpenGL GLX extension not supported by displaymichael@Michael-Lenovo-IdeaPad-Y570:~$