I've had a couple cases where quest stacking helped me just fly through a level. It's a lot tougher on my main since I don't usually know which quests are easy to do at the same time, but I just hit on a few that are really obvious. Bloodmage Lynnore and Bloodmage Drazial are a couple NPCs in the Blasted Lands who give you a pile of quests that require you to just kill a bunch of critters nearby. The two Bloodmages are standing next to a bonfire in Dreadmaul Hold and all the creatures that drop the quest items are south of them and along the road that curves around to Nethergarde Keep (I'm writing from the Alliance point of view but this all applies equally to Horde
Lynorre gives The Basilisk's Bite and Vulture's Vigor then Infallible Mind and Spiritual Domination. Drazial gives you Snickerfang Jowls, The Decisive Striker, and A Boar's Vitality first, follwed by Rage of Ages, Salt of the Scorpok and Spirit of the Boar.
The idea is that the first time you turn in a the quest you'll get a buff and an item with one charge that repeats the same buff. For example when you do The Basilisk's Bite the first time you get a buff for +25 intellect for an hour. Similarly the others each give you a buff on one stat and a single-use soulbound item that you can use to get the same buff again.
After you finish each of the quests, there's another quest available which will give you another item but no experience. The second set of quests can all be repeated for the item. All-in-all there are five quests you can get that will give you experience and six quests that you can repeat for the items. The items do give a nice buff, but the drop rate of the items you need generally doesn't make it worthwhile in my opinion.
So if you pick up the whole set of five quests at the earliest level possible, level 45, then you can get a total of 4700x5 = 23,500 XP - somewhere around one fifth of the XP for the next level.
I had a hard time at level 47 killing the level 51 scorpids that I needed to. Toss in the wandering high level lost ones that immolate your hide and the fact that your opposing faction is out there doing the same quests and you can be out there a long time basically grinding on these quests. So I had a peek at the auction house to suplement the stuff that dropped. I killed a bunch of stuff myself, but there's no way I would've spent all that time just blowing stuff up for these quests. Especially when there's not even an inn anywhere in the Blasted Lands for me to log out at.
With that in mind I thought it'd be handy to total up all the goods you'll need. Then if you're in a situation like me where you've got your mid-forties main out in the field working up to 60 and your mule in the aucion house handling the money, then you can make a shopping list and keep an eye out for an easy XP boost.
Here's the list of parts
I got on to these quests because I saw a bunch of brains in the auction house for cheap and I wanted to know what they were used for. So I bought those and a couple pincers and killed the rest. None of the items are soulbound or quest items so there's no problem buying your way through these quests or mailing parts to an alt if you've got a high level character that can cruise through farming what you need.
Another thing about the quest rewards here is that each buff will replace any other buff that's already active. So if you have the +25 spirit buff and receive the +25 intellect, you'll lose the spirit one. To get the most out of the buff, you can head north to the Swamp of Sorrows and grind for a while. I did that a couple times because I didn't want to waste a buff but saw a couple high level hordies running around (and I'm some kind of gank-magnet).
Finally, you can only carry one of each of the quest rewards at a time. For example, if you already have one cerebral cortex compound then you can't turn in the quest to get a second cerebral cortex compound. If you're only after the XP then that doesn't matter since you don't get XP for the second one. If you actually want the item I'd recommend you just sell the parts on the auction house and buy yourself something pretty.