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)
Third: Don’t skimp out on consumables. I recommend getting 20 flask, 20 Drums, 40 Runes/Food, and about 20 pots. If you are Horde I HIGHLY recommend about 40 Goblin Gliders, it will make your life so much easier. Consumables are pretty cheap right now (on my server at least) and I do recommend you stock up. Ask a friendly mage for some food before you start leveling so you don’t have to waste your buff food.
Professions: This may seem strange for a leveling guide, but the first opportunity you get, pick up both the Mining and Herbalism gathering professions. Not only will this allow you to track many various quest objectives on your map, but you also get a fair amount of experience whenever you gather. I'd put it at around 10% of a quest turn-in per gather. Since most of your wasted time in WoW is travel time, stopping to gather ore or herbs can keep your exp/h rolling as long as you don't go too far out of your way. You should only gather if it's convenient, and on the way to an actual quest objective. Don't make gathering your priority.
Through out the game, the quest log will be capped to 20 many times. The guide makes use of maximizing the total amount of quests you can accept at once. Because of this it is important to follow the guide exactly and only accept quests if the guide tells you to so you won't run into issues not being able to accept a quest. If you do accept a quest that is not listed in the guide, write it down because you may need to abandon it in order to keep following the guide if your quest log is full.
Fourth: This is very important; do not vendor the gear you obtain while leveling. There is a new mechanic introduced in BFA called the Scrapper which allows you scrap your gear for trade goods which will be very important for rep and AP grinding (will talk about it more later). Usually after I clear a zone, I will head to the scrap bot and scrap all my greens/blues to clear my bag for the next zone.
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The zone you start in isn't under your control, in as much as it's pre-selected for you based on your race. But for leveling purposes we want to make scaling work for us as much as possible. That means sticking to the lower level zones. Even though scaling will increase the level of the quests and mobs in those zones, the layout and design of them is built for a player with no mount. So you'll blaze through them and gain more exp/hour than higher level zones which assume you DO have a mount.
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)