On January 30, 2018 Battle for Azeroth was made available for preorder, coming with the unlocking of the four first allied races. With the announcement of the Collector's Edition and release date for Battle for Azeroth on April 5, 2018,[3][5] Legion was the first expansion to be offered for free with a preorder.[6] Battle for Azeroth is the first expansion to launch in the Americas, Europe, Taiwan, Korea, and Australia/New Zealand at the same time.[1] Battle for Azeroth became the fastest-selling World of Warcraft expansion ever.[7]

Waypoint System with Taxi Feature - Works similar to TomTom, clicking the Quest POI in your Objective Tracker will automatically place a waypoint for the quest. The waypoint arrow will turn green if you enter the selected quest area. You can also create your waypoint by CTRL + Right Click on your map and you can also link them together by holding SHIFT + Right Click on your map.

At 98 you have the option to drop what you're doing and return to your capitol city to start the quest for your Legion Artifact weapon. However, I do not believe this is necessary to do right away unless you're struggling with kill speed in your current gear. I would recommend waiting for a natural pause, such as hearthing to repair and clear your bags. From 100 to 107 you won't even really fight mobs that often; only just enough to clear the way to any treasure you need to pick up.
Make sure you have the two Flight-Path toys purchased, as this can save you time. It won't have EVERY flight path, but will have major locations for faster travel if you need to switch zones entirely(which you will). If you buy the toys on one faction, it unlocks for the other faction as well. So don't spend double the gold! These toys can be found at the heirloom vendor(Undercity for Horde, Ironforge for Alliance)
Leveling used to be a treat in WoW. New, unique abilities drove you forward and made getting past the horrible grind worth it to some extent. Now it just feels like everything is set to grind. More than it used to be. Once it was tolerable, rewarding even, but now you grind to unlock an allied race through reputations barely connected to most of these races, only to have to level up that character to get their unique armor set (compelling you to not just boost your character and instead suffer through the slog of ANOTHER group of alts), then you get back to current content and grind some more with a random chance to drop the item you need to get your ilvl up to finally enjoy the single warfront available (as I write this).
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.

Dugi’s leveling guide is the leveling addon I’ve always used to level as fast as possible. It will show you where to go and what to do in-game, speeding up the leveling process. This is optional though, as the game already does a pretty good job at showing you where to go at all times. The addon does, however, come with a couple of other nice features and perks.

When you have completed your third zone, you should be nearly max level, but you still have work to do! Head back to your faction hub and complete the last foothold quest to unlock your fourth mission table champion. After, you will want to do your newly available invasion missions for your faction. This quest chain is one that will be time gated by reputation levels earned with the 7th Legion for the Alliance and iThe Honorbound for the Horde. Complete as many quests as you can and then finish up your leveling with side quests you may have left in previous zones as necessary.
Mythic+ has some similarity to raiding, but it’s designed for just five players, instead of the 10-to-30 that can make up a raid. This mode tasks players with clearing the game’s dungeons, but with specific changes that make them harder, scaling up until it’s impossible for groups to finish. Players receive a keystone in their inventory that assigns them their dungeon, one of the game’s 10, and gives the dungeon a level. This determines how difficult enemies are and what effects might make them harder. If the group completes the keystone in time, they get a new, higher-level stone; if they, don’t they get a lower-level stone.
Even if you don’t have flying in WoD be sure to get the WoD handy notes. If you have flying simply fly and pick up all the treasures that you can and do bonus objectives. Not every single piece of treasure will be worth it though as some are hidden in caves or are just hard to get. So if you find yourself spending more than a few minutes finding a treasure, just skip it and move on, there is plenty of treasure to go around.
And, of course, there are plenty of in-game addons you can use to make your adventures in Azeroth even more efficient. Try checking out Azeroth Auto Pilot, which points you where to go, handles all your NPC discussions and quest turn-ins, and automatically sells your junk gear (among other features). Deadly Boss Mods is still a must-have, so you don’t die in dungeons quite so quickly. Pawn helps you tell whether the gear you’re getting is actually an upgrade or not. World Quests List gives you a much better way to see what’s happening in your zone (and sort for the rewards you’re prioritizing).
Elixirs serve the same purpose as Flasks, but the buff expires if you die. You'll only use these between 1-70, generally speaking, until you can start buying Flasks which persist through death. Try not to die, and carry extras just in case. If you can't find an Elixir with the stat you want, look under scrolls, since they count as battle or guardian buffs as well.
Heirlooms. At the bare minimum you make sure all the armor pieces upgraded to 110, since this is the lion's share of your experience increase. Neck and Ring upgrades are secondary, but still important due to being able to enchant them. Trinkets can't be enchanted, but you'll want them at max as well, if only to save time by not having to worry about replacing them as you level.
There’s a lot of #content in the new expansion. It’s mostly locked behind a variety of gates, some of which players can’t even control. Warfronts are the best example. You might assume joining a Warfront is a simple matter of opening the group finder. Wrong! Warfronts are only accessible for a week at a time. It’s not precisely clear how often that week will come around (it’s based both on a time gate and on the opposing faction gathering resources) but, as of right now, it looks likely to be available just one week out of every four.
Allied races aren’t much better. You can only earn them by grinding out reputation. It doesn’t take that long but casual players will need to invest a couple weeks primarily to it, and focusing on that grind will take you away from others, like earning gear. Each race has its own grind, but the things you do to work through it remain the same, adding to your boredom.
DoTs: Assuming that the mob is going to live for certain periods of time, it is worth it to place DoTs up. If the mob lives for at least five seconds, cast Pain. If it lives at least seven seconds, cast VT. The ideal time to cast DoTs is when it would be replacing Flay, but you can substitute Blast if the mob will live longer. Make sure to cast Bolt on cd though!

Taking Herbalism and Mining as your two professions can be a lucrative way to level with the very stable and high prices of herbs and ore thus far in BfA. While rested, each mining and herbalism node will award nearly 4,000 experience points which makes it so that four to five herbalism or mining nodes is equivalent to one quest. If you have the  Sky Golem mount for herbalism, you can pick herbs without losing mount speed. There are also enchantments for gloves that will increase the speed at which you can process the herbalism and mining nodes. These can be made rather easily by an enchanter and should be relatively inexpensive on the auction house. These enchants will pay for themselves with the time saved while herbing and mining. Remember to continue completing mining and herbalism quests while leveling up to increase the yield of each node making your herbalism and mining leveling experience exponentially more lucrative.

101+ At this point you'll start taking a penalty on quest/bonus objective experience for every level past 100. Treasures, however, remain worth their full value all the way through level 105, at which point they'll be worth something like 15% less, and then another 15% more when you ding 106. I recommend that you stop questing/gathering entirely, and collect all the treasures you can stomach, only avoiding any annoying to reach ones inside caves or behind elites/rares(where scaling does bizarro things).
XP Potions: The Elixir of Rapid Mind and Elixir of Ancient Knowledge no longer drop. They can be bought on the AH still but are way to expensive for it to be worth it. The Excess Potion of Accelerated learning is available from your garrison quartermaster but only works for level 91 to 99. As of Patch 8.1 you can also buy the 10% XP Potion Draught of Ten Lands.