Essence Font should only be used to get the HoT effect out in order to quickly burst single targets through the double Gust of Mist hits. Cancelling a cast of it when your party has the HoT effect rolling on all of them will allow you to use Mastery-proccing spells faster for burst healing on any target that needs it. However, continuing the channel can be the right call in specific situations, normally when you need group healing while you yourself are moving. It depends on the situation at hand.
Heirlooms are special pieces of armor that level up with your character, so they have stats that are always respectable for your character’s level (up to level 110). Heirlooms that fit the head, shoulder, chest, legs, and back armor slot offer an experience bonus. The total bonus is 45 percent if you have them all. It’s also possible to acquire a ring that adds another 5 percent, but you must win a fishing contest acquire it, and that can take some time.
For Horde, Vol’Dun the area captures the feel of Battle for Azeroth is both the hardest and continues than other areas in the game. It can get quite frustrating in this area so better to go it early when you can utilize your old gear. The next two areas are highly dependent on personal opinion – Nazmir is filled with mobs and Zaldalar is one of the easiest zones.
The zone you start in isn't under your control, in as much as it's pre-selected for you based on your race. But for leveling purposes we want to make scaling work for us as much as possible. That means sticking to the lower level zones. Even though scaling will increase the level of the quests and mobs in those zones, the layout and design of them is built for a player with no mount. So you'll blaze through them and gain more exp/hour than higher level zones which assume you DO have a mount.
The power of items depends on their item level, which you can see on the item's tooltip; the higher the level, the better an item will be. However, the Warforging and Titanforging system - introduced in the Warlords of Draenor expansion - means that almost any item of Uncommon, Rare or Epic quality has a chance to ‘forge' to higher than its base item level: Warforging is up to +15, Titanforging is up to +45.
If we consider a group of players, the squad has three main roles: “tank”, fighter and healer. Warrior can be a great “tank”, in other words, the defender. “Tanks” are very hardy and can absorb a large amount of damage, but their main task – divertenemy’s attention from more vulnerable members of the group. Magicians, mentioned above – are the characters that cause a lot of damage. Priests, whose vocation is to heal the suffering, cannot inflict significant damage, compared to other classes, but their role is difficult to overestimate, because they help colleagues to survive, using their knowledge of healing.

Tiragarde Sound and Stormsong Valley are both fairly similar zones, but Stormsong is a little longer, meaning that you might not even have to finish it to hit 120. Tiragarde also comes with a bit of a warning. There are plenty of side quests to do in the zone after you finish the story quests. Don’t do them. It’s not worth the time or the experience you’ll get.