Faceroll usually refers to something that is so very easy it takes very little planning or strategy. It comes from the idea of rolling your face over the keyboard to issue commands instead of carefully picking them. It mostly gets applied to formula specs or group compositions that are so dominant, it is perceived that the game is too easy in those combinations. It started with Arena team types and then migrated to refer to the ease of some raid instances (mostly in Wrath of the Lich King) given over-geared groups.
WoW Insider Edit
Faceroll: A class, spec, or composition said to be very easy to play. Ex: "I could faceroll my keyboard and get gladiator with that composition."