I've been having a lot of fun throughout World of Warcraft's Legion expansion, but I'm coming from the experience of having played it since the beginning with a heap of high-level characters to romp around with. For many Legion is their first crack at WoW (or their first MMO), and several changes to the leveling system have a lot of people up in arms.
In particular, between 2014 and 2016, it was possible for dedicated players to generate quantities of gold that were previously impossible to obtain, and have not been possible to obtain since. The Warlords of Draenor expansion was the first time the game included nonplayer character followers that players could send on missions to collect resources, items and other useful stuff, including gold.
Usually it's best to avoid a lot of instance runs. It is too difficult to account for bad groups, and some instances are too far away with too few quests. Since the experience is very good, it's a good idea to clear nearby instances which have a lot of quests once (Gnomeregan, Deadmines for Alliance). Distant instances with few quests should be avoided (Deadmines for Horde). Low level instances can be cleared by paying or befriending a single high level character. However, since patch 3.3 a new Dungeon group finding system has been implemented, as well as a direct port to dungeons (when entering the group you get ported to the dungeon and back to the same location if you leave it) and thus the travel time to dungeons has been greatly reduced. This makes it a good idea to complete quests or grind while you wait for a group to become available.
The Horde 12-25 (Barrens + Stonetalon Mountains) guide has been rewritten and revamped. In addition, there has been numerous tweaks to the speedrun route to make things faster and easier to follow. Also, in case you haven't noticed I am now adding about 40% more info per step and redoing all the images. Unlike with my previous vanilla guides, I am now giving a brief explanation of where to go and what to do with each step along with any other helpful tips.
The problem currently is that a previous patch introduced a much smoother paced leveling system, though it was definitely slower. Many actually enjoyed the tweak to the overall progression system but ever since patch 8.0 went live, many MMORPG lovers are reported that is unnecessarily difficult to level through some of the older content drops, which was an unintended side affect according to Blizzard.
I used fully upgraded and enchanted heirlooms; missing only the fishing ring. Leveling from 1-20 took ~3 hours, at which point I immediately turned on War Mode for the duration. I rarely encountered enemy players, and when I did they almost always left me alone. But the few times I did get ganked, the total cost in time was insignificant. Maybe a total of 5 deaths from ganks in the entire 40 hours.
Blizzard upping the health of creatures in those early zones has had a profound effect on how low my health bar gets. Even decked out in a full set of heirloom gear, my little fury warrior hasn’t one-shot much of anything in her 22 levels. Mobs that survive longer hit more often, hence my dangerously low health in the screenshot below. Players have to be a little more mindful now. That’s a good thing.
Most heirlooms only work in a limited level range in their default form and must be upgraded to work at higher levels. That costs gold, and you’ll need around 30,000 gold to outfit yourself with a fully upgraded set (counting only those that give you bonus experience). You may need to spend some time on a boosted character to acquire the gold you need to outfit another character with heirlooms, but it’s well worth the effort.
Lastly: The zone order I recommend for Horde is Vol’dun- Zuldazar- Nazmir. Vol’dun is definitely the most challenging of the bunch and having the use of your Legion gear and leggos will smooth this zone out. Nazmir is the easiest of the bunch which is nice when things start to get a little grindy past 116. For Alliance, I recommend Drustvar- TS- Stormsong. Same logic as the Horde side, in my opinion Drustvar is the hardest and Stormsong is the easiest for when it gets grindy. I would recommend you waiting to do any of the War Campaign until you are capped because it just takes you out of the way. I also recommend not killing a single rare mob or picking up a single treasure because in my experience it is just not even remotely worth it. The gear scales up on the rare mobs so its smarter to keep them till 120 anyways. After you are done leveling, I do recommend taking the time to do all the quest at the Faction Outpost in your enemy territory. They do provide rep for the Honorbound which helps. Most of them are quite easy. If you do not level in Warmode, you will usually hit 114 from your first zone, 117 from your second, and cap 120 about 2/3 in your final zone.