Legion seems like the expansion that takes two steps forward, and one step back. There's a lot of good things going on, like M+, the AP/AK dynamic, and world quests. But the over-use of RNG and gating is killing the good that those mechanics are bringing to the table. The lack of flying up til now is also a major black mark against the expansion, although that could theoretically change if content in 7.3 and onward uses flight instead of trying to deny that it exists.
Demon Hunters are...a mixed bag. I feel like they're only there as fanservice. Mechanically speaking I think the game would have been better off with another mail-wearing class with three full talent specs. Spellbreaker, Warden, or possibly Engineer/Tinker would have all worked decently well. Warden would have especially fit the expansion thematically. DH's only getting two specs is LOLtastic. Just shows there wasn't enough theme or lore to justify them in the first place. They even had to steal Metamorphosis from warlocks.
I love the idea of world quests, although there should be more variety, and there should be more degrees of world scaling for them, similar to M+ or Diablo 3's torment levels. The PVP balancing and separation from PVE was something that should have happened years ago, so that's a welcome change.
Crafting in the expansion has been pretty damn good, although I could wish there were more cutting-edge recopies for higher iLVL gear.
Legendaries, IMO, are a complete and utter failure of implementation. They should have been used to add or modify talents or class abilities. Lateral options instead of being such BiS items where you suffer a massive performance failure if you don't get blessed by the RNG with the right one. I would have preferred having them obtained through class-specific quests with high-level difficulty, as we're seeing now with the solo challenges. That way at least if they're going to be BiS, everyone who can perform at a high-enough level to need them would be challenged to get it, proving that they deserve it. Getting a legendary from kicking squirrels every day is just fucking stupid.
Story/Lore wise I think things are finally getting interesting. I've always wondered what the Legion was doing while we were screwing around in panda land and back-to-the-future alternate timeline garbage. Also loving that we've got an expansion that isn't orc-centric for once.
All in all I feel like Legion is kind of a wash. It's not terrible, but it's not great either. I'd put it above WoD and Vanilla, but below everything else.