-Artefact weapons, best borrowed power system in the game with a SPEC SPECIFIC unique ability and a brand new talent tree to upgrade with an AP system that, for all it’s flaws, actually got fixed.
-Artefact Weapon Appearances, of all the borrowed systems legion is were people come back and farm the appearances. Those appearances were some of the best in the game and they struck a perfect balance of availability and difficulty to get. Every type of content got some to unlock, so whatever your prefered content was you could get some with some unique colors for the best of the best players in each categories.
-Orders halls. Who cares about covenants ? Every single class get their own base of operation, full of famous characters of this class and little secrets like the dark ranger headpiece for the hunters or the pet reward of the shaman mini game.
-Class templates in pvp, disliked at the time but all the tears about gear imbalance in pvp makes me think many miss it.
-Peak Spec identy: remember when all spec had to share a talent tree? Well with Legion everyone gets their own talent tree with great spec reworks like pirate rogue or survival hunter which finally becomes something more than “like all the other hunters, but bad.”
-Great lore: we fight a villain we actually care about, the mess that was TBC lore gets fixed without retcon by simply giving more context to it. Or even Suramar, what a great zone. We finally go to Argus aswell, like can you see how crazy this is ? “Here is an ENTIRE PLANET, have fun.”
-Tons of content: 77 days patch cycle, with intermediairy patches that actually gave content. Like the class mounts for example, people still go back to unlock those on their alts.
-Some of the best raids in the game’s history, we finally go to the nightmare, we see Illidan disenchanting Gul’Dan in the Nighthold, tomb of sargeras which pretty much had two end bosses and Antorus with Argus. Oh Argus, you ascend to the thrones of the pantheon, surronded by literal gods while you fight a god too with a banger of a theme music in the background.
-Introduced m+: casually creates a brand new form of high end play, something which hadn’t been done since TBC added arenas.
-Allied races: Introduced in 8.3.5, legion was so generous in content than in a half patch it introduce a brand new concept of playable race with four of them.
-Actual worldbuilding: through the stories of the order halls we got to see the entire world of Azeroth and see how it changed since last time we went there. For example, the whole thing with Bolvar was introduced in the DK campaign.
-Class campaigns: speaking of which, Shadowlands ? 4 campaigns. BFA ? 2 Campaigns. Legion ? 12. 3 times more than SL and 6 times more than BFA. And every single one of them is about your class, the most important thing about your character, with a story tailor made for it filled with moments only a true fan of their class can appreciate. Like in the dh one when Kil’Jaeden appears before the Slayer in his giant ghostly form, offering the dh to join him in a scene reminescent of his pact with Illidan back in the Frozen Throne.
-Demon Hunter: probably the most demanded class since the game released legion finally delivers, with it’s record of popularity through highs and lows the dh class is probably one of if not the most succesful new class added to the game. And what an entry it did ! A brand new zone offering a new perspective on the Black Temple raid and a story filled with lore and cinematics.
-Removed chi and stances from mistweaver monk 
it’s like rewatching shows with story lines leading nowhere and a crappy ending you know is coming. But maybe in a few years who knows 