I have to say....I LOVE this game!
My favorite expansion before this one is Wrath of the Lich King. Mainly because it had a substance to it that I enjoyed. All around story, visually, and questing was fun and immersive.
Pandaria has that same thing. A vast continuous land mass with lots of stuff to do and see.
So here is my brief review.
The story is OK. I don't find it compelling, especially for Alliance. Searching for Varian's boy doesn't really give me a great reason to invade another country.
The world I find really interesting. Every zone has it's own feel to it (at least the ones I've seen so far), and are rich with good and bad factions. Generally the vista's and the buildings are really beautiful and I find the lighting really awesome, especially inside of buildings and dungeons.
The quests have been good. There are some new unique quest types that I have seen that involve moving you to another location and having you follow someone and do things with a vehicle bar. Not new mechanics per se but they appear to be presented in a different format. At least they feel original to me. I can't think of anything to compare them to in previous expansions. Other than that you have the usual quest types.
The use of phasing has been subtle and well placed.
I spent the first two levels questing with a friend the entire time and I do have a couple of con's I'd like to share. The duo questing worked really well for the most part but we did find that we got stuck in certain situations. We had one experience in Valley of the Four Winds where we were stuck in combat until we both logged out and rebooted our computers. We had a few experiences where we would go to turn in a quest or begin a quest and all of a sudden find ourselves split up or running after an NPC. I am not sure how Blizzard could make those better except to give you some sort of notice that you will be moving as soon as you start this quest, or give you the "speak to so-and-so when you're ready to go". Those seem to work well.
So far I am enjoying this expansion immensely. One week in and I am 5 bubbles from 90. Which for me is a new record. Normally I enjoy meandering through quests, but feel compelled this time around to jump in and hit the ground running. I hope you all are enjoying it as much as I am. :)