My toon was a monk, a class that gets extra experience for an hour at a time. I picked at it more than I have in the past, so I benefitted more from the 150 percent "rested" experience bonus. If I wanted to run a dungeon for quests, I fired up a high-level character on a second account and dragged my little toon through them. And having leveled dozens of characters to 100 and 110, I knew every zone and every objective.
Most classes in World of Warcraft start at first level, but Death Knights and Demon Hunters are exceptions. They start at level 55 and 98, respectively. This alone can shave a huge chunk of time from your leveling experience. Of course, it only works if you want to play a Death Knight or Demon Hunter, but they’re both solid classics that can dish out damage or tank in a group.
EXCEPTION: If you have Mining and only use Jewelcrafting to craft stone statues (for additional in-combat healing), then this may be a useful tradeskill. However, like Engineering above, it should not be a leveling focus and should only be used to assist with power leveling. Doing any more than this will become costly and should not be undertaken during the power-leveling process.
Also get the Heirloom Mount. Being able to have basic riding speed from the beginning will save you a lot of time if you're starting from level 1. Allied races start at 20, so this is a non-issue for them. It's important to note that Pandaren can not use this mount until they finish their starting zone and pick a faction. Being Horde or Alliance is a requirement of the mount.
Fourth, beginning in Mists of Pandaria with the Halfhill farm, and continuing in Warlords of Draenor with various garrison buildings that produced resources, Blizzard has been making trade goods more plentiful and easier to obtain. In Legion, herbs, fish and enchanting materials were so abundant that a very small population of gatherers could supply more raw materials than the economy could consume.
As always, I will monitor and update this guide as new information comes in. If I've gotten anything glaringly and completely wrong, just tell me with as much data as possible. The idea is to find the fastest possible leveling method, NOT to argue about how good or bad individuals are. If you have hard data, post it! If you have an idea, let the thread know so we can test it!
In 2015, Blizzard started letting players buy gold from each other using WoW Tokens, to try to control the process and mitigate damage. “Time is money, friend — but sometimes one is harder to come by than the other,” Blizzard says on the official page. “Now World of Warcraft players can use the WoW Token in exchange for game time or Battle.net Balance!”
Dungeons still give too much experience. Yes, I'll get some hate mail for this. But due to the way dungeons -- especially at some levels -- scale, along with experience buffs, the fastest way from 1-110 now involves grinding one dungeon over and over and over again. It's the equivalent of the South Park "Make Love, Not Warcraft" episode, only instead of killing boars, you're killing endless worms in Ragefire Chasm or gnolls in The Stockades.
Usually it's best to avoid a lot of instance runs. It is too difficult to account for bad groups, and some instances are too far away with too few quests. Since the experience is very good, it's a good idea to clear nearby instances which have a lot of quests once (Gnomeregan, Deadmines for Alliance). Distant instances with few quests should be avoided (Deadmines for Horde). Low level instances can be cleared by paying or befriending a single high level character. However, since patch 3.3 a new Dungeon group finding system has been implemented, as well as a direct port to dungeons (when entering the group you get ported to the dungeon and back to the same location if you leave it) and thus the travel time to dungeons has been greatly reduced. This makes it a good idea to complete quests or grind while you wait for a group to become available.
The WoW token is very simple: You pay Blizzard $20 for a token, and then you can sell the token on the in-game auction house. A player with gold can buy a token and redeem it for a month of WoW subscription time or for $15 of Battle.net balance, which is like a gift card credit that can be redeemed in WoW or other Blizzard games such as Hearthstone and Overwatch. You get their gold; they get your cash, or at least most of it.