Tag-Boosting - The fastest way to level

Once you've brought your second or third character to level 70 levelling a new character turns from an adventure into somewhat of a tedious grind.

And even if you do like the endless grind, the quest story lines that you were quite possibly following with your first character and maybe your second are starting to look a little bit old and dull.
You probably also tried different zones that you haven't tried before but even those quests are looking awfully familiar after the 3rd time round.

So now you can either dig in and pray you don't go completely insane before you hit 70 or you can ask yourself the question:

how the hell do I get to 70 faster?

Opinions may vary on this one. Some say get people to boost you in instances. Others say go quest alone, a few AoE specialists swear by grinding certain groups of mobs in certain locations and other still say you should group up and have the high level character carve you a path through your quest mobs.

I say regardless of what people say take a step back and look at the mechanic of XP gain:

1. Questing/grinding ungrouped gives the most xp per kill
2. When you group you split the xp over the amount of group members that are present (the split is more even when you are all of about the same level)
3. If you group with a high level character xp suffers from diminishing returns meaning that you get far less xp
4. If someone outside your group helps kill your mobs xp also suffers from diminishing returns if the other player is of significantly higher level than the mobs you're killing (5+)

Just looking at these facts we can draw the following 2 conclusions:
1. Not grouping is better than grouping
2. Partnering up with someone in your level range (+10 levels max) yields more xp than someone whose significantly higher than you.

Time to introduce my druid. He just recently hit level 60 and is questing away in Hellfire peninsula. The level 63 mobs aren't a significant challenge but they do take quite some time to eliminate.
He calls in a friend's help to speed things up. The friendly rogue is level 70 and is capable of chewing through level 63's as if they're cardboard.

Because however the mobs are gray to the level 70 the xp gets cut by more than half giving my poor little druid very little xp. This is still better than doing it alone because you amass quest items quickly but it doesn't really make a significant difference.

So now we move my level 60 druid and my 70 rogue friend to terokkar. The mobs here are higher level and pretty much orange to my druid and green to my rogue friend.

We now notice a significant difference in the XP distribution when being partied. My druid gets a nice chunk of xp but this is still diminished by the rogue friend in my party. It's better, because you can't kill high level mobs as effectively alone but it's still not significantly faster than just questing/grinding in hellfire.

But the 2 conclusions we drew previously are still valid. The rogue and the druid are fairly close in levels (10 is about the maximum we're shooting for) so now we need to take the next step and break the group relationship.

So we ungroup. The druid attacks a mob and the higher level rogue shreds it to pieces after the mob has been tagged.

Since we are no longer grouped we don't get diminishing returns from our partner and since the partner is fighting green mobs there are no significant diminishing returns based on outlevelling the mob.

So now the druid gets 'full' mob xp and benefits fully from the increased killing speed of the higher level buddy. You can substitute multiple buddies if you like to speed up the process even further.

To give an example: At level 60 in terokkar forest the above mob-tagging tactic gives you approximately 1200xp per kill and your killing speed is that of a level 70 tearing through low level mobs. This translates to about a level per hour but scales quite well if you can get more buddies or buddies that can aoe kill mobs by the truckload (or introduce rested xp etc.).

To summarize:

1. Get a friend or multiple friends that are no more than 10 levels above you.
2. Find an area where the mobs con green to your higher level buddy(s) and orange / reddish to you (mobs must NOT con gray to your higher level friend(s)).
3. Do not group (do not stay in the same party)
4. Attack a mob once (1 shot is enough to tag it)
5. Let your higher level buddy(s) kill off the mob
6. Loot
7. Rinse and repeat steps 4 - 6

This works better with some class combinations than others for obvious reasons but is still a good 2x faster (at the minimum) than regular questing/grinding. Combine this tactic with available quests in the area (those may be hard to find due to your relative low level though) and rested xp and even on a bad day you'll be levelling at a greatly enhanced speed.

Voila, Powerlevelling made easy.