MoP's launch was very good. Leveling was great and the continent was beautiful. Yes, the expansion had problems with the rep grinds, but aside from that it was very solid. Patches 5.1 and 5.2 were amazing, only 5.3 onward had a loss in quality and content, but SoO was a great raid and everyone wanted to off Garrosh. I remember the subscription charts back then. While MoP was not able to increase subs, it did keep people subbed longer, way more than Cataclysm or WoD were able to do.
MoP is my favorite expansion, but I started playing in Cataclysm, so I can't really rate WotLK against it. But questing/storytelling and even raiding seem more solid in MoP than WotLK for me.
I think most complains about MoP come from: (1) People not liking the theme/pandas; (2) The 5.0 rep grinds, which were fixed two months later when 5.1 came out; (3) The poor content 5.3 onward; or (4) being tired of the very long content drought having nothing to do but Timeless Isle.