I thought Cataclysm was pretty bad and easily the worst expansion. Haven't tried WoD but I've heard it's bad in large part due to a content drought.
The problems seem to compound with each recent expansion. The early zones in the expansion tend to be fine for questing but after awhile it starts feeling uninspired. Also creating a new alt is boring due to how dead the game is from lv1-70 (if not higher). Meanwhile the endgame....look, I loved raiding for a long time. But I just got burned out, and that's when I realized there really wasn't anything else to do. Outside of grind for mats.
Ultimately BC and WOTLK will always be the best expansions. Lots of content out the gate and a variety of shit to do. I was never into PVP but loved how it was incorporated into PVE with both expansions. In BC, Isle of Quel'Danas was fun; it was a grind yet the inclusion of opposing faction players made it feel fresh, plus the instance and raid were fun. WOTLK had Wintergrasp which was amazing; I entered the battle everytime I was able to, and even stayed up late to do it just for a chance at gaining access to the raid. Those Vault raids were one of the few things that could be done relatively quickly and give you a nice reward. It was also a great way to meet folks on the server. You quickly realized who the best players in your faction were, who was a good raid leader, who was best avoided, etc.
