As you're leveling, I generally recommend you just follow the natural flow of the progression through each zone. They're laid out fairly reasonably. But don't be afraid to hop around, especially if you're near a quest objective. You shouldn't actually need ALL of the zones listed to reach 62. Pick the ones you like best. It's much more important to simply be as focused as possible and flow from one quest to the next.
The expansion brings a major change to the PvP ruleset on each realm. Every realm by default only allows players to attack NPCs in the open world; players who wish to engage in world PvP now have a setting called "War Mode" that can only be toggled on or off in their faction's capital city (i.e. Stormwind for the Alliance and Orgrimmar for the Horde). While in War Mode, players have access to new talents and abilities, as well as a slightly accelerated rate of XP gain.[10] Characters with War Mode activated are only able to see other players with War Mode, unless in their own capital cities.
-Bugs- There aren't too many glaring bugs anymore, but you will certainly encounter some. The pathetic thing is that many of the bugs you'll encounter were reported repeatedly during Alpha/Beta and still weren't fixed. Some bugs were discovered after release and players were able to abuse game mechanics without repercussion unless they were one of the more serious offenders. This gave some players a significant advantage over others because they used exploits and at the start of an expansion, that is the time when it's most impactful.

These guides are completely automated and work in-game, so you can see step-by-step instructions as you play. The guides automatically move to a new step as you complete the quests. You are told where to go i.e. the questgivers for accepting and turning in quests along with where and how to complete the quests. Zygor has more steps and details, which would be more suited to beginners. Dugi condenses the info into fewer steps, but still has all the necessary questing info.
Island expeditions are very easy to complete while leveling, particularly before hitting Level 116 when your legendaries are still in effect and your secondary stats have not been squished by leveling scaling yet. Island expeditions can be completed in 5 to 10 minutes relatively reliably while also acquiring some of the pets, mounts, and Bind-on-Equip gear available. There is also one weekly quest to do one island expedition, so for slowly leveling alternative characters, be sure to complete at least one island expedition each week.
Today, I finally found out what the real problem was. I have always had the "Enable Zone Map" feature off, but noticed that I had "Enable Auto Zoom" still enabled. I assumed if you turned the map off, the auto zoom would just automatically not do anything. Turns out, every time I moved, the auto zoom was still collecting information and 'zooming' a map that didn't exist. I've turned it off now and my game is perfect - not a single slowdown. The difference is dramatic.
War Mode: War Mode can be turned on at level 30 through your talent window, and will give you 10% bonus XP. You will, however, be attackable by players of the opposing faction, but changes have been made so that all players have a fair shot in a fight and even low-level players can kill much higher level players, and at least not get ganked and one shotted. Note: As of patch 8.1, you can get an additional call to arms bonus with war mode on if your faction is underepresented. 

Currently, it is believed that the War Mode rewards will change depending on which faction is more heavily prevalent with War Mode activated. The Horde have typically had more participating in War Mode, so the reward has been increased for the Alliance to incentivize more participation in War Mode. These rewards could potentially be increased to both the Horde and Alliance to incentivize both factions to higher amounts of War Mode participation.
While leveling, try to make use of the new Battle of Azeroth experience gem, Insightful Rubellite, which gives you +5% experience. Equipping one of these requires a socket in your gear, so try to hold on to a piece of Legion gear with a socket or acquire one of the Legion Legendary rings since every class has one. Each of the rings includes a socket. You can acquire one of these rings rather easily by doing World Quests in Legion zones and acquiring 1,000 Wakening Essences that can be used to purchase a legendary ring in the Legion version of Dalaran.
-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.
Update: The reason for less experience from gathering is due to how Blizzard revamped professions and broke them down by expansion. In order to get full exp from gathering you'll simply need to find a profession trainer for the relevant expansion. (WoD you just mine or herb the first one ypu come across and the game gives you a scroll for learning WoD gathering).
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.
There are other in-game guides out there such as Xcelerated, but it doesn’t even come close to what Dugi and Zygor offer.  Although I’m obviously a fan of Dugi Guides, I have given a fair side by side comparison of each guide, based on facts and what is offered by each. I’ve included many screenshots to give you an idea of how each guide looks and works within the game. I will do my best to keep this as up to date as possible, with price changes, guide updates etc as I know how annoying it is reading outdated reviews.
That’s because armor of higher level has higher Azerite power requirements than lower level armor. That means a shiny new piece of Epic gear will have better stats but, unless you’ve been diligent about your Azerite power grind, won’t have as many traits unlocked. You lose options as you earn better gear. Eventually, you’ll grind out enough Azerite to earn those traits back, but you’re left with limited customization until then.