I just spent 6 hours trying to figure out why the heck I wasn't able to successfully start an "X" session on my raspberry pi 2 with archlinux installed. It turns out that, for some reason, the board kept defaulting to the `mali` driver every time a graphical session was started. I don't remember, at this point, whether the mali driver was installed as a dependency or whether it was installed deliberately due to my ignorance of the proper GPU driver for the raspberry pi 2. In any case, I thought it might be a good idea to, at least, document my experience here for future reference. Uninstalling the mali driver resolved the issue.
NOTE: Maybe we could expand a bit on the great Installation instructions to include the proper setup of the Graphical desktop.