2. Achieving. There are many pet battling achievements in game, and following the natural trail through the Taming [Continent] achievements up to Pandaria, and then on to the Spirit Tamers, Beasts of Fable and the Celestial Tournament was an epic journey for me and for many others. It's well worth doing. The Celestial Tournament is still, IMO, the pinnacle of PvE pet battling. However, WoD and Legion and BfA have added more battling achievements.
The home page for the guide says that using it is like having a leveling guru giving you constant advice. Having used the guide for a quite some time now I’d say that’s pretty accurate. You’ll always know what to do and where to go next, automatically. Remember all those questions in the in-game trade chat, asking what to do or where to go next (for whatever level?) It won’t be you asking those questions.
Similarly, when earning by volume, do not overload the AH with your items. Basic supply and demand: too much supply will make your wares too common and unwanted. Furthermore, with great demand and a cheaper price, you may end up starting a price war with your competitors, with them actively trying to undercut and outdo you. Moderate the amount of items you're selling on the AH to small batches, refilling them only when sold out.