In a word: magnificent!
Admittedly, I've only done two or three questlines in the very first zone. But so far it has been Blizzard at the very top of their game. Fun, enjoyable quests. An interesting storyline. Killing lots of Horde. Gorgeous scenery. Lots of excellent small touches.
The SI:7 mini-questline was superb.
Now, it is a bit crowded with tons of people running around. Truthfully, I think Blizzard intended to use the Cross-Realm Zone techniques to split up the crowded zones into multiple ones. But since the outcry against CRZ is harsh, they've just let the zones be.
But still, it's the same as every expansion at release.
The gear rewards are interesting. If it's a green reward, you only get the reward appropriate for your current spec. But if it's a blue reward, you can choose the item for whichever spec you prefer. I think that's a reasonable way of keeping your dungeon set up to date if it is different from your questing set.
I really hope Blizzard can keep this up for the rest of the continent. If they do, it will be--by far--the best expansion of WoW.