First: I started in Vanilla, not Classic. People who started the game in classic play less than 3 months and probably don't know MoP at all. And for me it's not one of my favorites, it's my absolute favorite. 8.0 had an reputation-issue, and it's still one of the top 10 stupid things blizzard made, putting valor gear behind reputations (and was quite quickly hotfixed to relax the requirements; not sure if they removed it later alltogether); but beside of this, it was the best expansion blizzard ever made.
I think the biggest issues some people have with MOP are simply one thing: PANDAS!