Everyone calls it the worst expansion for the most part. I used to think that, but looking back, Cata only had 2 major issues. 1 was not enough raid bosses after tier 11, and number 2 is Dragonsoul being terrible. If tier 12 would have had 3 more bosses, and Dragonsoul hadn't been rushed it may have been one of the best. I used to think MoP was better by leaps and bounds, but now I think I can rank them about equall with MoP having a very slight lead.
Cata would have been better than MOP is dragonsoul hadn't sucked so much.