My favorite part about MoP is the fact that I had no interest in it so I did not purchase it. This allowed me to drop WoW and not really have any desire to play since.
Pokemon
Pet Battles and the "farm", now I really want housing in game even more then before. Also the new hunter tracking challenge pets are great, now they just need to add more stable slots so I don't have to abandon some of my already full stable of rares. I will also say the new zones look great, really fun to explore.
Favorite thing so far is the lore/humor and the pet battles, it brings a lot of interesting fun into the mix.
My favorite is the bet battles.
My favorite part of MoP has to be the amazing job they did with the zones. They are a stunning sight to see. First expansion that made me want to run at Ultra 24/7 and deal with lower FPS instead of at medium.
The Environment and music :) And of course the Cloud Serpents ;) The Pet Battles is awesome as well, gah there's just too much fun stuff in MoP! :D
Gotta be the pet battle system!
item upgrades its brilliant to give certain items that extra little kick :D
Farming in Valley of the Four Winds. AoE looting and pet battles are definitely candidates for the first position though.
I love how fast the made the Monk level. Now they need to give the same type of thing to every class. Leveling alts sucks all the way to 90. I'm an altaholic, and have 10 lowbies that I'm actively leveling and working on leveling 10 85's to 90 at the same time. with 3 already at 90. some help with the lowbies would be awesome. my eventual goal is to have i of every class every spec at 90. I have 14 horde the rest are alli for now, but it'll even out I'm sure.
Pet battles has kept me entertained for hours which makes it my favorite part of MoP.... Oh and AoE looting saves the day when farming those bolts of silk in scarlet monastery :D
My favorite part has to be all the lore; it's tied into the exploration, dungeons, raids, and questing better than any previous WoW expansion. And having a faction exclusively dedicated to telling stories just makes it even better.
Removal of stance requirements from warrior's ability :]
My favorite part of Mists of Pandaria so far is the artwork. The instances, particularly in Dread Wastes but really in general, are some of the most beautiful and richly imagined since Ulduar. The zones too are extraordinary. It's a beautiful, complex and well thought out expansion as well as being fun.
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The storyline about mantids, being a loregeek i liked all that lorestuff they wrapped mantids into.
I love the pet battles! They are tons of fun and add a new facet to the game that is a good social activity!