Here is a quest in Howling Fjord for 68+ level: If Valgarde Falls... - Quest - World of Warcraft
Upon taking it endless waves of monsters will start to run at north gates of town.
So idea is simple: You need to 1 hit monsters and NPC will finish them off.
You will get ~2.5k exp(depends on mob type) for each on lvl 69(heirlooms, rested).
To maximize exp per min you will need to stay as close as possible to friendly NPC so they will do melee attacks(that do ALOT more damage then their ranged attack).
As 70 Shadow Priest i was able to get 500k exp in 10 min just by spaming Vampiric Touch on every mob.
Also you could use Guild Banner for increasing EpM even more. (I didnt use it)
SS of me getting exp at lvl 71 without rested:
Attachment 9421
Update:
Just tryed to move into Disc and spam Holy nova into all of spawning mobs(Just running into them and spam nova then return to gates where mobs finish them)
This way was MORE efficient than Vampiric Touch.