After playing wow for so many years now, I have to say, cataclysm felt to me the worst period of time in the games history.
The alterations to the world. The attitude of appealing only to hardcore players instead of the wider community. The changes made to the lore. The divided max level zones only access via portals. And so many other problems.
I' hated playing though cata, and around the time found myself slipping off onto other mmos and games to take a break from it. The only saving grace felt like the final installment, and even then the raiding was worse then anything before.
Overall, hated cata with every fider of my being. Where as Mists, well still feeling the pinch of what cata had caused, mists feels like what expansions like wrath and tbc once had to them, giving more access to players to do stuff, even with the boring grinds (which lets face it tbc had that same feel), it just feels so much better.