This is new and comprehensive guide to levelling exploits.
There are too many explois to list here and many are better shown than explained, but here are some of the main points:
1) Fel meteor + Goblin Glider can allow low-level players to effectively, even in no-fly zones. Dramatically increases movement and therefore levelling speed.
2) "Method Zero" - my default method for levelling up to 90. Form a group between a levelling and high-level character on the same account. Take both into a dungeon. Run through the dungeon on your hi-level toon and aggro but do not kill all bosses, log-in on lowbie, log back in on hi-level toon and kill all bosses/trash you've kited. The lowbie should receive quite a lot of experience for dungeon completion.
Method zero, especially with a fast toon and skystep potion+bear tartare is generally faster than any other form of xp grinding, though as you outlevel a dungeon LFD with very low que time, a good group and available quests can become superior.
While this is probably one of the most powerful techniques I've ever developed, applying it optimally is not trivial. Expect a bit of frustration.
3) Proving grounds farming. Essentially you join proving grounds through a class trainer then teleport out of the instance. Return at any time and start the countdown and your health is restored to 100% and all your cooldowns reset allowing for incredibly fast xp grindng. I've published this before a long while back for gold farming purposes but I've never mentioned how powerful it is for levelling. This is most useful from 70-80 (farm converted heroes outside the front of icc) and legion itself where dungeons aren't an option.
4) Demon Invasion ability glitching. There are numerous demon invasion quests which give you some OP ability to farm mobs fast. In several (possible all now?) cases these abilities vanish when you fill up the bar. By joining a raid you suspend quest progress and can farm xp indefinitely (or at least till the invasion ends). Note: players below aren't getting invasions currently, however this is a glitch rather than bliz policy change.