EXPLORE TWO FABLED KINGDOMS – As a champion of the Horde, travel to Zandalar to persuade the trolls to lend their naval might to the war effort. As a defender of the Alliance, sail to the seafaring kingdom of Kul Tiras, home of Jaina Proudmoore, and rally its inhabitants to fight for your cause. Once you’ve successfully recruited one of these proud kingdoms to your faction, infiltrate the enemy’s newly claimed territory.
When you have completed your third zone, you should be nearly max level, but you still have work to do! Head back to your faction hub and complete the last foothold quest to unlock your fourth mission table champion. After, you will want to do your newly available invasion missions for your faction. This quest chain is one that will be time gated by reputation levels earned with the 7th Legion for the Alliance and iThe Honorbound for the Horde. Complete as many quests as you can and then finish up your leveling with side quests you may have left in previous zones as necessary.
I like his recommendations 2 level 1-20 zones to get to 20, then do all the 2nd zones since they are designed for walking (Horde: Northern Barrens, Silverpine Forest, Ghostlands, Azshara, & Hillsbrad Foothills if you need a little more. Alliance: Westfall, Loch Modan, Darkshore, Bloodmyst Isle, and if you need a little more Redridge Mtns, or Duskwood). Don't even bother with the higher level areas, too spread out. Finishing up a Monk now and I had done all these zones before the update, the running between quest and the 50% experience cap is driving me crazy.