It's odd, but for me, the killer feature of Mists is your farm at Halfhill.
I've played games which had player housing, and similar cosmetic structures, and I've never really done anything with them. I've bought the land or house, maybe added one decoration, and then never bothered with it again.
So why is Farmer Yoon's farm so different?
I think it's because the farm is useful.
I like logging in and harvesting vegetables to level cooking. Or planting crops for various quests. Or planting crops to get raw materials for professions or even Motes of Harmony.
It's like you don't need to farm mats anymore, you can just "farm" them. Very zen.
I like upgrading the farm, and making it more useful. I like that there's a "rhythm" to the farm. Log in, harvest today's crops, plant tomorrow's crops. Simple, and yet feels just right.
I even like getting the cosmetic features like the dog. It's like player housing, but making it actually useful instead of just cosmetic was the element that put it over the top.
Excellent work on the farm and the Tillers, Blizzard.
I hope the next expansion sees an expanded farm system, maybe near Stormwind, that is more integrated into the game as an evergreen feature, rather than just an expansion-only feature.