I don't know how it goes for Horde, but right now there's a Invasion in Tiragarde Sound and there's a World Quest called "A Carefully Laid Trap", on which you have to kill 3 rares.
On my 117 Druid (with heirlooms), I am receiving 13590 rested exp with each rare, so everytime they die, you receive 13590 x 3 = 40 770 experience. Without Rested it's about 7k each rare.
They respawn every 45 seconds, aproximately.
So basically what you need to do is join one of those WQ groups in LFG (you can also create it) and stay there until you're the leader. You do not receive exp while in a raid, so your group needs to stay with 5 people or lower. When you become a leader and more people try to join in, you get a message asking if you want to convert the group to a raid. Just click no, and then you can afk while people come and join your group to kill the rares for you.
I've gained 100k exp while typing this post, there's always people joining, killing the bosses and abandoning the group, leaving space for new people who need to kill them.
It's obviously not the fastest method, but you can literally leave your PC unattended and do whatever you wish while receiving good experience.