A Mythic+ dungeon is a much more difficult version of the original, and loot doesn't drop directly from bosses; instead, a chest will spawn at the end containing a number of items that will increase depending on how quickly you cleared the zone. The level of this loot will increase based on the difficulty level (Keystone) of the dungeon you completed, starting at 345 and scaling to 370 for a M+10 or higher.
Class abilities were reworked dramatically across the board, with some still needing a serious balance pass. That left many players not only feeling underpowered, but having to learn how to play their characters all over again. Bosses in large-group raid dungeons that were easily killed each week by experienced guilds of players suddenly became challenging again, or worse, completely unkillable, halting forward momentum on old content for a good chunk of the high-end player base.
Compare that to the previous Warlords of Draenor expansion, which wasn't a great expansion overall, but still managed a prepatch that offered a pet, a title, a new version of an old dungeon with new quests and achievements and loot, a series of ultrapowerful weapons that could be passed on to alts, and a whole host of in-game system improvements ranging from a new way to find groups to a new way to collect pets, toys and appearances.
At the end of Legion, the titan Sargeras was imprisoned, but not before he plunged his sword into the planet Azeroth. This not only devastated a massive area (much of the desert zone of Silithus is now cracked and scorched), but badly wounded the gestating titan inside. While the heroes managed to ablate the worst of the damage (at the cost of the majority of their artifact weapons' mythic power), the world is still wounded and bleeding a substance called "Azerite," which has great magical potential. The Horde Warchief, Sylvanas Windrunner, attempts to consolidate Horde power on Kalimdor and gain a monopoly on Azerite (which is primarily found at the southern end of the continent). Her campaign to do so, the War of Thorns, formed a pre-launch event for the expansion and ends with the major Night Elf holdings on the continent seized or (in the case of their capital city of Darnassus) annihilated. The Alliance makes a retaliatory strike against her home base, the Undercity, which was formerly the human capital of Lordaeron. This succeeds in driving the Horde out, but before leaving Sylvanas saturates the area with disease and toxins that render it uninhabitable. With this tit-for-tat military exchange, the Horde has almost complete control over Kalimdor, while the Alliance has near-total dominion over the Eastern Kingdoms. With further conflict inevitable, Battle for Azeroth takes the two factions to the continents of Kul Tiras and Zandalar to recruit new allies in order to turn the tides of war.
Players’ relationships with the NPCs around them have been hit-and-miss over the years. One of the original issues with Thrall, the Horde’s original Warchief, is that players began to refer to him as “green Jesus,” because of how infinitely powerful and infallible he was. But he’s been gone since the Warlord of Draenor expansion, and with the death of Varian Wrynn and Vol’Jin at the start of Legion, the old guard hasn’t been there to guide players the same way as it used to be.