This guide was inspired by a post we made the other day.
To gain rep with all faction reputations, first get the tabard for each (locations are contained in the video), then go to shadow labyrinth in auchindoun, terrokar forest, outland. Set dungeon difficulty to heroic.
Go to the end boss room and kill all the mobs other than cabal summoners. (there's an advanced method to do thiswhich is quite complex and needs to be seen to understood).
The summoners spawn mobs very fast. Each kill gives you 5 faction rep.
I believe this is the fastest possible way to grind rep for a faction.