This is indeed very powerful abuse/exploit, but remember that with powerful exploits comes a risk for getting banned. Yes you can get banned for mob-tag-powerleveling. Based on little forum searching the lenght of bans varies from few days to permanent bans, depending most likely on how much you abused and how much you have exploited in the past.
Anyways... For mob-tag-boosting and for regular legit boosting.. This is the route I used with RAF and occasional random boosting on retail wotlk, so far this has been the best route I've seen. (I've tested this route on Warmane up to Sunken Temple so far, seems to be exactly the same here)
Level 1-10: Quests
Level 10-20: Deadmines
- Run to van cleef, run back to the instance door, AoE all. Rinse and repeat. (Btw, in case it's not obvious, have your low level friend / your alt wait at the instance entrance while you're pulling)
Level 15-25: Stockades
- Same as Deadmines, Gather all mobs and AoE at the instance door. The downside of Stockades is that it's very small and you will reach instance cap very fast.
Level 25-45: Scarlet Monastery (Library, Armory and/or Cathedral, Cathedral giving the most experience)
Library - Same as Deadmines, pull everything to the instance door and AoE. When pulling the last boss, do some damage to him until he goes immune and casts his AoE, if you don't he might oneshot your friends lowlvl characters with it.
Cathedral - Do a circle, gathering all mobs, bring them back to the instance door and AoE. Tip: When you get to the church door, run and pull Commander Morgraine (or w.e. his name is), this pulls most of the church mobs with the boss, speeding up pulling a lot. Another tip: Enter first with your high level character and kill the first Myrmidon of the instance or else the myrmidon will aggro to your under lvl38 friend/alt and kill him.
Armory - never really did armory, I'm guessing gather all and AoE.
Level 40-50: Sunken Temple
- Turn left from the start of the instance, go up the spiraling stairs, you will find a room full of dragons, pull dragons, kill dragons, go back and repeat.
Level 45-58: Stratholme (Some people like scholomance as well)
- Do Baron runs.
Level 55-68: Ramparts - Blood Furnace - Shattered halls
- Pull what you can, kill.
Level 68-X: Utgarde Keep
- Pull what you can, kill. I usually stop running Utgarde Keep and move to questing at some point. (That point is usually reached as soon as I'm getting way to bored.)
Some Classes are better than others with boosting. Prot paladin is most likely the best, druid coming next. What class your high level is affects the effectiveness of boosting a lot. For example: On my prot-paladin and balance-druid (both on full wrathful pvp gear) on Retail I could pull everything in all instances listed above. However on my priest (Same gear) I had to do 2-3 pulls on some instances.
The eagle-eyed readers might've noticed that the level to move to the next instance is a bit flexible, if you like an instance do it a bit longer, if you hate an instance, move away from there asap.
Instance caps:
You can do 5 instances per hour. If you are fast enough, you might get a message: "You have entered too many instances recently." This is when I usually take a break or do one battleground on my main or log bank-alt or whatever.
Notes:
Zul farrak (level 35-45): Used to be godly, blizzard nerfed this, avoid this instance. (except, if you want the gear from here, then you can run until you get the gear you desire and move back to cath or sunken temple after that)
- If you decide to go here, remember that Shadowhunters use dismounting shot on you as soon as they spot you, knocking you off mount and Witch doctors have to be single target killed or else their greater healing wards easily outheals your aoe. 1-2 healing wards dont do much, but 15 of those makes them outheal pretty much everything.
Gnomeregan: Avoid, annoying AoE from troggs + they seem to just stand still instead of following you making large pulls nearly impossible.
There's plenty of other instances that work as well.
Shadowfang keep seems to be popular choice
Uldaman works afaik
Maraudon is fairly good
Blackrock Spire is very common as well
For the Horde:
Instead of Deadmines and stockades, use whatever the instance is that is located in orgrimmar, use wailing caverns and the other low lvl instance at the sourthern part of barrens. (or Shadow Fang keep)
Edits: Formatting to make it easier on the eyes.