They have revamped the recruit a friend system and added a bunch of goodies!
There is an exclusive Zhevra mount that you can get on one charecter once your friend signs up for a full acount! I haven't seen pictures yet but that is something I'd like to have.
While you and your friend quest together you get triple xp. You also have the ability to summon each other anywhere in the world once an hour. Another cool thing is that for every 2 levels your friend gets they can "give" a low level toon of yours a level.
Almost makes me want to set up a second account in a different name but then I wouldn't be able to merge them later.
Details are here:
Blizzard Support