People always whine about the current content the most in WoW, for some reason. Then an expansion after that they start admitting it had good parts, and then an expansion after that it becomes "good old times" and sometimes achieves legendary status. Happened to basically every expansion so far, will probably happen to the current one as well. For MoP specifically, some of the raids are considered the best ever, and world content patches are also highly praised (timeless isle, thunder isle).
And no, MoP was not hated in general. Some idiots were shouting in the beginning about there being Pandas in it - mostly because some people always have sexual connotations with everything and think that if there are playable animals in the game it's supposed to fulfill some sexual fantasy. It's unfathomable to some that people might just want to play Panda or Vulpera for reasons other than sexual arousal.