Introducing the Pandaren is not the problem. Theming the entire expansion around them...that is the problem.
Everyone asks "Why is this stupid, when Walrus men, Blood Elves, Space Goats, etc was not?". It's because "Wrath of the Lich King" was about Northrend, and Arthas, and the Walrusmen were just a bit of silly side flavor, and they were called "Tuskarr" and not "Walrusians".
Now imagine it had been a Walrus themed expansion, called "Shores of Walrusia", and starred the Walrusians, on the continent Walrusia, and had gobs of Walrus themed content. Think people might have complained a shark was jumped? I think they might.
Come up with a real expansion and just shuttle the Pandaren in there as a playable race, and the fuss would have been minimal. And now I need to try and think of a way to say "Doing it like this was just pandering" without it coming across like the world's worst pun. Thanks Blizzard.