Not much about the content in Legion is really drawing me in, it's all about the mechanics changes and quality of life stuff:
-They are mixing up a lot of classes that felt stale or downright shitty to me in WoD. Top of the list being Disc Priest, while Shadow Priest, dps Shammies, Warlocks, DK and Warrior tanks also look pretty interesting.
-The new transmog system is also going to keep me occupied for a while. I've tried to collect a full set of tier armor for every class since vanilla, and limited bag space was usually the biggest obstacle.
-Challenge mode dungeons. I tried to put together a group of friends to do challenge modes in WoD on a casual basis, and the whole experience was just miserable. We expected everything to be tougher, sure, but forgot that our gear scaled down as well. So we jump in assuming a certain level of dps and healing that we didn't actually have. Then we run into all kinds of arbitrary restrictions that only make sense when challenge modes were supposed to be PVE Esports: Can't mass rez, can't switch out party members, can't change talents/glyphs. Finally after beating our heads against it and respeccing/swapping around our class composition we beat one and get a silver. We go turn in our daily and get a piece of shit 640 loot, when we all have mostly 645-655 gear just from garrisons; 640 filling in the rest of our slots from LFR. From what they've shown of challenge modes in Legion, they've fixed pretty much all those problems.
-Artifacts, I'm curious about, not excited. They could be Legion's garrisons, i.e. an incredibly cool and fun looking mechanic on paper, repetitive and ultimately bad for the overall gameplay in execution.
Storywise I don't give a fuck anymore. I checked out in Cataclysm when they took the 2nd most fearsome, evil, and intelligent antagonist in Warcraft history, Deathwing, aka Neltharion the Earth-Warder, the Dragon Aspect of Death, and disregarded most of his backstory, presenting him mostly just as a stereotypical giant angry monster videogame boss.