So today, I was triple boxing and running my RAF alts through instances. I got a pally and a hunter to level 21 without even doing their level 10 quests.
Then I decided it was a half decent idea to get pet training, so I went and did the quest. The pally was with me, and when I turned in the first section of the quest (like, tame an elder boar in durotar) I was very pleased to be rewarded with 1500exp (4500 with RAF). When I got the next section, I looked in my quest log and saw that the quest, which is obtained at level 10, was level 21, the same level as my toon.
This exploit is useless if you are not leveling by being run through things, and somewhat useless if you're not on RAF. With RAF, I imagine the whole quest chain would net you about half a level at level 30.
Pretty situational exploit I know, but I'll let you guys decide how useful it is.