That would be AoE looting and the new talent system :)
The Monk class, it's so much fun that it's now my main.
I love the AoE looting aswell as the new zones including Dungeons, raids and BGs
It has to be the detail in the new mesh (characters, npcs, and mobs)
As a Rare Hunter I enjoy the exploration of a new contitnent and finding as many rares as I can. The thrill of finding that one creature or item, such fun.
I really like the new raids and the loot system in LFR .
Cant describe how much I love the AOE looting. Im not sure how we went so long without it.
My favourite par of MoP are the new zone, I totally love it. Then the Pandaren XD. The monk class. The aoe loot. Damn i love that expention
no hunter range and in next patch no standing around to cast shots
Pet battles , naming pets , new raid content , new lfr system , new talent trees.................. to name a few things :)
It's hard to select only one thing, when there are many pieces to enjoy. I greatly appreciate how improvements have added to the overall enjoyment, but by far my favorite is Pet Battles. I expected it to be a nice distraction, but it has become much more that just that!
My favorite part about MoP is the rep-grind. You actually have to do something again to get rep instead of mindlessly farming heroics with a tabard.
The tillers faction. I was concerned it would be some silly farmville graft to WoW and it turned out to be so fun a faction to work with.
I love the story line of Yu'lon.
Loving raids and new loot mechanics, cant wait for 5.1 upgrades
Jade Forest is the best part of the whole game.
My favorite part of the MOP is the scenarios, and that it only takes 3 people to get them done. It is something new and different and very refreshing.
I like the Asian, architecture and themes for the entire expansion. Love them.
I'm very happy to see that all aspects of the game are being addressed and not just PvPing or PvEing. Professions and faction rep are front and center.
Environment: The music, the atmosphere (especially in the Jade Forest, love it) and the quest lines.Mechanics: the new talent "trees" and aoe looting.:-)