Comment # 2 on bug 953666 from Ken Gramm

Yes, it probably is more of a "packaging" issue than a XFCE issue; and starting
it with xhost.... might have been OK for a third party app that you had to
manually add from the build service, but as steam is now an official package
which comes with the installation media, it should just work out of the box. 

And if you pick XFCE as your desktop, it doesn't.

Also, I've tried this on two other machines and I get the same results every
time.

KDE, GNOME, and LXDE work great.  XFCE crashes with the previously stated
error.