-Azerite Gear- This has been the worst experience for many players as it was advertised as offering the player many options for customizing their character but all players do is Google "Icy veins (Spec/Class) azerite ranks" and select the best one. This is expected and really only gives players the illusion of choice as they're always going to pick the best one. This could have been done far better by not offering necessarily just damage increases in traits but offering some of the Legion Legendary item effects in Azerite slots. You know, certain Azerite traits would be better for PvP, some for raiding, some for 5 mans, some for soloing old content, etc. The other problem with Azerite gear is that it's locked based on you grinding an endless amount of Azerite for your neck piece (basically the stuff that increases the level of your neck piece). So when you get an item that should be an upgrade, it really isn't because you don't have all the traits unlocked. This complaint about how horrible this system is could go on forever, but I'll just leave it at that, horrible.
Heirlooms: Heirlooms are the best source of bonus XP for leveling. All slots can be gotten pretty easily now, except the rings. The time you need to spend to get the rings if you haven’t gotten a level 3 shipyard in WoW is way too long, so getting the rings is generally not worth it. Having all your heirloom gear will boost XP by 55%. Generally, you want heirloom gear with crit on for leveling as you want to kill mobs in a few hits as possible
As late as possible. Early on, you’ll want to follow the missions in Battle for Azeroth right up until you get the option to send a follower on a two-hour long quest. As soon as you get there, stop doing the War Campaign missions. You can come back to them once you have reached something like 119 and a half and finish leveling with those if you want. If you don’t use these missions to cap off your leveling journey, then they’re the first thing you’ll want to do once you reach max level.
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.
I highly recommend actually staying in the 20-60 range zones until you ding 62. The reason for this is that despite getting a slight penalty in experience for being over-level, the advantage of having flight will more than offset this. That, and WOTLK and TBC zones are the slowest part of the leveling process. Minimizing your time there as much as possible is worthwhile.
-Global Cooldown- Almost all spells are now on the global cooldown. This has slowed gameplay down quite a bit and you won't get to use the full potential of your spells cooldowns. For example, you cast a spell that increases your damage for 15 seconds, after the global cooldown and you're able to cast again, you now only have 13.5 seconds to make use of the remaining time. There are far too many spells on the global cooldown that shouldn't be.
You should try and quest efficiently - pulling several mobs if you can handle it, using your burst cooldowns while questing to kill everything faster. Many people would think that a 10-minute cooldown like Bloodlust is useless in open world, but you can do a "kill-15-mobs" quest in like 30 seconds if you pull a lot and use it. A free quest every 10 minutes is not that bad.
If you’re a returning player, consider paying for a month’s worth of playing time—a mere $15—and using the game’s newer Class Trials to see if you still have that spark for higher-level play. While you can also go the “Starter Edition” route for a free, limited romp through Azeroth, you might need a little more than the newbie zones to help you decide whether you want to play through the game’s latest expansion.
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.