Legion or any expansion will never be better then WoTLK or it's predecessors simply due to the community falling apart with Xrealm, LFG, LFR nonsense.
However Legion has addressed every issue plaguing the game starting with WoTLK.
Mythic+ Dungeons fixes a critical mistake made in WoTLK of watered down Heroics. They are now more challenging (not as hard as TBC Heroics still imo) that can scale as the game goes on. I would rather M+ be more dungeon crawly though then just a speed zerg, makes it more puggable, but Blizzard wants to incentives us playing with friends.
PvP Talents bring back a form of PvP progression. This is far from a perfect system, I'd still rather gear being locked out by rating, but it's something.
World Quest make the world feel alive again. Sadly we still don't interact with people the way we used to. I just have an addon that auto finds me a group in a quest area, and then I leave w/o saying a word.
Professions were addressed initially, they still need an overhaul to be more interesting like in other MMOs. Right now crafting is a joke. but M+ increased the demand for potions.
I actually love how they bought back attunements also. Overall Blizzard seemed very sadistic in Legion, we whined about not having anything to do in WoD so they threw everything at us in Legion.
The RNG loot and Legendary systems has its problems, but it gives you a reason to do stuff. Go and do Kara, EN, ToV and fish for Titanforge Gear and Legendaries, and since your not only relying on a drop, but also relying on Titanforging you don't feel like you need to do it. Casual players shouldn't complain. I can understand if Mythic Raiders can get frustrated.
I think people are having a hard time adjusting to the fact that you can't really "finish" a character for a given Tier, and just gear another one or wait for the next patch. But I love how I'm allowed to LOVE MY MAIN and always having something to get on with when I log on it. It hasn't been this way for me since TBC and I haven't been this addicted to the game since that time (sadly with far less time to play). I've barely had time to PvP this xpac which hasn't been a thing for me since TBC.
In previous expansions I'd have friends with 5+ characters fully decked out that I couldn't even distinguish which toon was their main. I'm glad this World of Alts nonsense we've had since WoTLK is gone. I have one alt with one spec at 37 traits, as opposed to my main with 44 Traits and 35 Traits on the other spec. How it should be imo. Alts are bigger issue for PvP imo.