Personally, my favorite thing about MoP is the lore and storylines. With the introduction of all of the different factions it is nice to see how they all intertwine with each other.
aoe loot and pet battles
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My favorite thing about Mists of Pandaria is the new quest mechanics and the revamping of the way you get epic gear
I like the new character models
My favorite part has got to be some of the mechanics on the new quests, and the Klaxxi in general.
AE looting by far. Pet battles and challenge dungeons too.
I resubscribed after a year and 1/2 for the story! (running 5-mans and raids are fun too)
My favorite part of MoP is the versatility of the specs. No spec is wrong, it all depends on the situation.
Unexpectedly, I am totally addicted to pet battles.
a reason to go back to old raids
Definitely AoE looting :D
Definitely my favorite part is AoE looting, and the recent 5.1 removal of "raid-group" requirement for pre-MoP raid dungeons. Makes farming old content for transmog gear extremely enjoyable :).