Hello thar,
It seems that faction change resets all your quests you've done on the previous faction, that means, you can do 'em again and get double xp, double money, double rewards! I know this is intended from Blizzard, but it still gives quite an advantage (f.ex. if you change faction while you're leveling through 70-80).
http://i37.tinypic.com/sfdwk6.jpg
Let's say you've done Amphitheater with an ally, then you change faction to Horde. You can get two imba weapons, and shred people at BGs !