At the end of Legion, the titan Sargeras was imprisoned, but not before he plunged his sword into the planet Azeroth. This not only devastated a massive area (much of the desert zone of Silithus is now cracked and scorched), but badly wounded the gestating titan inside. While the heroes managed to ablate the worst of the damage (at the cost of the majority of their artifact weapons' mythic power), the world is still wounded and bleeding a substance called "Azerite," which has great magical potential. The Horde Warchief, Sylvanas Windrunner, attempts to consolidate Horde power on Kalimdor and gain a monopoly on Azerite (which is primarily found at the southern end of the continent). Her campaign to do so, the War of Thorns, formed a pre-launch event for the expansion and ends with the major Night Elf holdings on the continent seized or (in the case of their capital city of Darnassus) annihilated. The Alliance makes a retaliatory strike against her home base, the Undercity, which was formerly the human capital of Lordaeron. This succeeds in driving the Horde out, but before leaving Sylvanas saturates the area with disease and toxins that render it uninhabitable. With this tit-for-tat military exchange, the Horde has almost complete control over Kalimdor, while the Alliance has near-total dominion over the Eastern Kingdoms. With further conflict inevitable, Battle for Azeroth takes the two factions to the continents of Kul Tiras and Zandalar to recruit new allies in order to turn the tides of war.
Buffs. Food/Flasks/Scrolls/Potions/Elixirs are a small, but noticeable, buff to your killing speed and survivability. If you have a max level character that can already supply you with cooking or alchemy, you'll be very well off, because the AH is a cesspool for this. I can't really give you specific foods or potions to look for, since each AH is going to have different supplies. Generally speaking, lower level foods are going to only last 15-20 minutes until you can buy the more powerful versions at 60+. Look for anything that adds versatility or your primary stat, and buy stacks for each level range(1-60/70/80/90/100).
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.
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.
Some people have made arguments that quest blues can sometimes be more powerful than heirlooms, especially with sockets. I'm not convinced. The simple fact that you never have to even look at quest rewards will save a non-insignificant amount of time over the course of leveling. Also of consideration is the value of enchanted gear never having to be re-enchanted.
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.
For reference, my previous best time abusing Method-0 in 7.3.5 was 32.4 hours(as show in the old thread). A difference of just over 8 hours; which isn't all that bad, all things considered. And I believe I made several errors along the way, which could be cleaned up in future runs. I believe that that gap could be significantly closed by using a monk's bonus, but I hate playing monk...so...yeah. YMMV