MoP always had a group of people that highly praised it. It had great questing, interesting lore, fun raids, and a unique area - as well as a new race and class. There have always been people that disliked it ("because pandas") but it wound up being more popular than the vocal minority that simply didn't like having a new animal race or Asian aesthetics.
It also looks good in retrospect, because it was smashed between Cataclysm and WoD. It actually was good, but it looks like a shining beacon compared to the surrounding expansions. It wasn't perfect - anyone remember the dailies? - but it was much more carefully made than other expansions.