Congratulations on reaching Level 120! Now that you have hit Level 120 we recommend looking at our Easy mode page and Talents section to learn how to play at max level. To get yourself prepared for the end-game content ahead, we strongly recommend this Battle for Azeroth Raiding Lexicon/Spreadsheet by Medivh. It is a gold trove of information that will help you when gearing up in Battle for Azeroth and more.
The idea of the expansion came up after the heroes of Azeroth dealt with so much external threats like the Burning Legion. In their own words, "so when we talk about what's the biggest threat in this world, is it the titan or is it the dwarf paladin that put a hammer in that titan's head"? As such, the enemy (at least during the early parts of the scenario) will be the opposite faction, a "vast army of world-destroying forces".
Heart of Azeroth: Our world is direly wounded. Players will be summoned via Magni Bronzebeard to Azeroth herself. She will gift players with a medallion called the [Heart of Azeroth]. Her life essence seeps from the world in the form of [Azerite]. When players wearing the medallion come in contact with azerite, this sought after resource becomes a source of power for selected pieces of armor, providing players a way to level them up. This feature replaces the artifact system of World of Warcraft: Legion. Currently the plan of development is to limit the upgrade system to 3 pieces of armor. Players will be able to choose the stats they want to better fit their play-style.
Warfronts are larger, 20-man challenges that take inspiration from the Warcraft strategy games. Players are deployed on the Warfront to collect resources, destroy enemy troops, construct buildings, and eventually launch an assault to take over the enemy base. These battles take 20 to 40 minutes to complete and, unlike Island Expeditions, don’t offer a PvP mode.
This farm almost did not make the list, since every YouTuber and farmer alike has beaten it to death since it first appeared sometime in 2012. There are so many videos on this spot, that it's hard to to stumble across one, even while searching for something completely mundane, like chicken wings or golf clubs. Anyway, you want to pull the entire instance without aggroing any of the bosses, and if your a skinner you an make slightly more gold by skinning the hides. Generally this farm is being phased out by the lvl 58 twilight outpost farm, and with good reason. It's better and does not require you to enter the instance. So more drops for you! Not sure, why anyone would pick this spot over the latter unless you were really interested in increasing you skinning level.
Also get the Heirloom Mount. Being able to have basic riding speed from the beginning will save you a lot of time if you're starting from level 1. Allied races start at 20, so this is a non-issue for them. It's important to note that Pandaren can not use this mount until they finish their starting zone and pick a faction. Being Horde or Alliance is a requirement of the mount.
As others have said, there are free alternatives... which is important to consider if the price of the Zygor Guides are a concern. If speed and convenience are more important, then they're worth it. Zygor will take you through the quests in the most efficient way possible, and their customer support and the speed with which they update the guides when changes are made to the game is impressive. I know I went from 80-85 a heck of a lot faster (in /played time) than my co-worker who doesn't have the guides. And we had a bet going to see who would make it first. For me, it was worth it. Your mileage may vary...
The upcoming raid, Battle of Dazar’Alor, tackles this problem by having the Horde and Alliance have two raid stories — and the opposite faction will temporary flip sides for flashbacks that show the whole story. The Alliance storm the Zandalar capital and gain a massive upper hand in the war. From there, the writers will have to balance a compelling Alliance story with the rising threat of the Old Gods (and, of course, the ongoing Horde story.)