So far (and I realize it's only just the beginning with no idea of the end-game) Legion is fantastic.
The things I've enjoyed the most:
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Classes are finally distinct. Each has a massive, different quest chain for their artifact weapon and class hall, with their own ways of fighting the incoming invasion; on my Druid (no spoilers) I've gained access to some pretty nifty areas, interacted with major characters from Warcraft's lore and certain NPCs greet my accomplished character with custom emotes. It's really great.
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Phasing is yielding fruit big time; gone are the days of 60 people on their mounts sitting on the quest givers while you were trying to click on that one pixel to get out of there, and yet you still actually see people around in the world doing things, as opposed to being locked up in your Garrison all on your lonesome.
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Some of the new zones are freakin' gorgeous too. The game's engine might be old now but they've really mastered it and it shows.
I did four 5-man runs yesterday (I got really sleepy by the end or I'd have done more). They seem pretty undertuned as most encounters are very forgiving to people standing in fires or whatever but maybe that's also due to everyone's gear - people play their mains on day 1 - being far superior to what Blizzard expects fresh 100s to be in the normal leveling process. The drops are pretty subpar, 685-700, so it looks like 104-105 is when we'll start actually switching from WoD to Legion-acquired stuff.
Oh, and I gotta figure out how to fit in my artifact weapon to the healing rotation. Btw resto druids can level just fine without switching talents, as long as they have Balance affinity picked up, but I don't know if all healing specs can do the same.