Once you reach an Alchemy skill of 475, you may begin a quest to learn one of three specializations: Potions,Elixirs or Transmutations. Being specialized gives you a chance to create an extra 1 to 4 (for a total 5) items while crafting in the field you have selected. For example, a Master of Potions may be able to craft more than one [Super Mana Potion] for the same material cost as one.
Do quests and complete the story of your first chosen area before returning to your capital to scrap all of the equipment items you acquired. You should be around Level 113 by the time you complete your first area. Before selecting your second zone, head to your faction hub ship to acquire your introductory quests to the mission table and your champions. Complete this introduction and then pick up the quest to establish your first foothold in enemy territory.
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.
Just like player item levels, the game's professions have become pretty bloated over time. Blizzard has made some updates to the leveling process before but Battle for Azeroth features a skill squish that should smooth things out considerably. Instead of having to level hundreds of points before you can start crafting Battle for Azeroth items, each expansion will have its own separate skill tier. In other words, you can start crafting Battle for Azeroth content right away, while the gear and items from other expansions will each feature their own separate counters. Your first Battle for Azeroth item will put you at 1/150 on the counter for the new expansion, no more grinding needed.
This top-end difficulty was missing entirely from Legion’s first raid, Emerald Nightmare, which was cleared by top guilds in around 11 hours. Meanwhile the first guild to clear Uldir’s hardest difficulty, Mythic, spent somewhere around 70 hours attempting to kill the raid’s eight bosses. Even after all that time, the guild, Method, still had members say they loved the raid, and that it was one of their favorite in recent memory.
The idea of the expansion came up after the heroes of Azeroth dealt with so much external threats like the Burning Legion. In their own words, "so when we talk about what's the biggest threat in this world, is it the titan or is it the dwarf paladin that put a hammer in that titan's head"? As such, the enemy (at least during the early parts of the scenario) will be the opposite faction, a "vast army of world-destroying forces".
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.
Meanwhile, in Zandalar, the Horde seeks to earn the trust of King Rastakhan so they can use his legendary Golden Fleet against the Alliance. To this end, they assist Rastakhan and his court in dealing with local threats in Zuldazar, fight maniacal Blood Trolls in Nazmir, and face off with serpentine Old God cultists in Vol'dun. Throughout their journey, the Horde gradually learns about an eldricth being known as G'huun, an artificial Old God accidentally created by the Titans and the patron deity of the Blood Trolls. The Blood Trolls and the Faithless Sethekk seek to free G'huun from his prison of Uldir so they can use him to rule Azeroth, and to this end they resurrect G'huun's champion, the C'thrax Mythrax, to destroy Uldir's seal. Rastakhan's own chief adviser, the prophet Zul, is revealed to be the Blood Trolls' secret leader and launches an armed revolt against Rastakhan. With the Horde champions' help, Rastakhan is able to defeat Zul and his forces, but not before Mythrax destroys Uldir's seal.
If you love running old content for transmog items, you'll love the new legacy loot system. Whenever a player enters a dungeon or raid where they are 10 or more levels above the content, the dungeon or raid bosses drop an amount of loot equivalent to what would have dropped for a full group when the content was current. That means players could earn loot for five people in dungeons and loot for 20 people after soloing a raid. The legacy loot system also drops other gear types, like cloth dropping for a plate wearer. It's now much easier to get the transmog gear you want, and you can also collect gear for your alts on the same run.