Yeah, unfortunately this happens to every expansion. I am a TBC-fanboy myself, but damn there was some boring shit happening back then.
I think every expansion bring in different people who is not satisfied with how the spec they are playing are doing, how it feels etc. So even though some people praise the class design in one expansion, it's just an opinion. I personally think Legion had the best class design(artifacts ftw) ever, but there are several that disagrees. Life is tough :P
Biggest turn-off for many with MoP was that it had pandas indeed. People didn't think that they were worthy of a story, and as playable race it just failed miserably. Going from Lich King to Deathwing, then Pandaria was a huge letdown for quite a lot of people. I am not saying the actual story was so badly made, but personally I think the trailer to patch 5.2 is the best thing with the expansion :P Best patch-trailer ever, which showed a good story. And that patch is what people seem to praise when it comes to MoP. It might overshadow what was wrong with it. Then we had 13 months was it, with SoO. So great, so great.