Some people have made arguments that quest blues can sometimes be more powerful than heirlooms, especially with sockets. I'm not convinced. The simple fact that you never have to even look at quest rewards will save a non-insignificant amount of time over the course of leveling. Also of consideration is the value of enchanted gear never having to be re-enchanted.
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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.
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.
That lack of storytelling agency may soon change with Patch 8.1, Tides of Vengeance, which is coming to Battle for Azeroth soon. While there are plenty of fun gameplay changes that should make the expansion better, one of the most interesting additions is that of choice. In an upcoming quest, players will be able to choose whether they want to support Saurfang or Sylvanas — essentially honor or evil — in the struggle for Horde warchief.