I've noticed that the biggest complaint with the latest expansion tends to fall into two categories.
1. "They ripped off Kung Fu Panda and now they're even kiddier than normal!"
2. "Pandas!? They're just blatantly pandering to little kids!"
Ignoring the fact that Pandaren predate Kung Fu Panda by about many years (and have, in fact, been requested by the fans for many many MANY years) , the core argument boils down to:
"The game has more colors than red, brown, and grit, and the style isn't realistic, therefore, it's immature and kiddy."
Ironically, people who complain about such things while praising games for having gore tend to be the kiddiest of them all....