Prior to update 7.3.5 it was far too easy to level up a new character. The experience needed to advance from one level to the next was too low, resulting in players out-leveling the story quests in each zone, moving on before getting the full experience. With low monster health and high player power (especially when players were outfitted in ridiculously powerful heirloom gear), creatures dropped like flies. Combined with easy experience gained by joining parties in the dungeon finder, the first 60 levels in the game flew by.
For a new player, this will only break the immersion and spoil the 'story' of the various great storylines found throughout the game. Veteran players may want to level fast but still, revisit old places to re-experience the game through fresh eyes. How is a player to level quickly without breaking the immersion of the game? Level smart. There are ways to quickly level that apply to any area of the game.
Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide