I found there were plenty of dark moments in the expansion. But I guess people getting tortured and having their souls ripped from their bodies is only badass if the Lich King does it. Or someone losing friends or loved ones doesn't mean anything. There was lots outside of the pandaren as well. 5.1 storyline was awesome. The pandaren, like most races have lots of cutesy emotes and whatnot, but they look tough when they fight. The female pandas have especially cool melee animations. I'm glad the pandaren are fat! A skinny bear is a sad bear. Just because someone is fat doesn't mean they don't have muscle underneath.
The zones look gorgeous. You don't need every zone to be dreary and dark to make the expansion have dark tones (though I freakin' love the mantid! Holy hell they are cool.) It doesn't matter what I say because people who didn't like the expansion won't like the expansion and will keep making the same comparisons they have since it was announced.