Without spoiling to much, the quests that I've seen people do and the storylines involved are a very heavy step up from blizzards usual tone, it's alot more mature and even darker in nature, and the story follows the zones.
The minute detail they've put in so far outshines WoD by miles, and FINALLY the mission table is being put to the grave.
This is the same bullshit every expansion gets, but from what I've seen and what I know of Blizzard this might be one of the best, it's a fresh start for Warcraft after all, now all the big bads from the RTS games are dealt with and done with.