My main problem with cata was the lack a clear design direction.
The 5mans were great, but then were nurfed to the ground, because there was nothing else fore casuals to do. Fireland normalmodes were great, but when we started heroicmodes, those were nurfed to the ground too leaving only ragnaros as a brick wall. The DS 5mans were unbelievable easy, DS was harder, but had one of those debuffs, which made everything till spine boring, while spine was undoable (for my guild) without the whole debuff.
It was just trying to fit to all player types at once and made all types (not everyone) unhappy in the process.
That and the lack of content, which was the result of the new old zones (which were great, but no one did them, because dungeons were faster, since you didnt need to travel all over the world everytime you leveled) and potentially all the wasted time they put into catas big feature "path of the titans".