We can't judge the expansion as a whole, we'll have to see how it unfolds. But at this point, it seems to outclass every other expansion.
Leveling:
All zones were enjoyable, the world is beautiful, there's plenty of high quality lore.
Dungeons:
The 8 initial dungeons are a blast (for me, specially maw of souls and halls of valor). There are 3 difficulties to climb. In a few days you can unlock two more dungeons and soon there will mythic+.
Class fantasy:
Needless to say, it delivers like no expansion has ever delivered. I can say i'm loving my class hall, my artifact weapons and my class campaign.
World content once max level:
There are world quests all around, the system worked pretty well (specially with the group finder tool). There is a whole zone to progress that is absolutely great. Suramar reminds me of the Island of Quel'Danas, but many times better.
Add to all that the revival of professions, Dalaran sewers, rares and treasures, and so on. The only mediocre thing about Legion is pvp, which has always been mediocre.
There's just so much to do, and not just anything, but actually quality content.
In compare, the other expansions had:
tbc: amazing leveling content, load of quality content once max level (get max flight, reputations, dungeons, heroics). It's the one that best compares to Legion, and it's outclassed hard.
wotlk: breathtaking leveling content, amazing zones, but little to do once max level and dungeons were too easy (initial raids too).
cata: amazing leveling zones minus twilight highlands, difficult engaging heroics (that didn't work too well with group finder), but basically nothing to do once max level
mop: so-so leveling zones but meh lore all around, ok dungeons, more world content, plenty of reputations that didn't work out so well, treasures, rares, and challenge modes. It was a solid attempt at max level content out of raids; it was perfected on future patches.
wod: so-so leveling zones, great cinematics, rofl lore, ok dungeons, boring as hell apexis world content, otherwise max-level content was sitting in garrisons. Just atrocious.
As you can see Legion gets leveling right as the first expansions did; as tbc, dungeons are great and relevant for progress; it also took what mop did right regarding world content and perfected it. And it had a smooth launch.