A Phoronix reader pointed out that as of the end of October, the xorg-server 1.17.3-1 update no longer uses setuid root by default. This non-root X.Org Server by default makes use of systemd's logind and libpam-systemd. It also needs a kernel DRM/KMS video driver, X runs on a virtual console from where it was started, and it now stores the Xorg.0.log within ~/.local/share/xorg/.
If you are trying to avoid logind or you're using a graphics processor not backed by a modern DRM/KMS driver, you will need to install the xserver-xorg-legacy package to let the X.Org Server run as root. Of course, the move to running the X.Org Server without root privileges is primarily for providing better security in case of the all too common X.Org Server bugs.