Basically Cataclysm is when The Dumbing Down started.. It is widely known that this is the point where the developers decided to tailor the game to attract casual players regardless of the negative impact on players who had already invested time learning the complexities of WoTLK.
Class talent trees and inherent abilities were completely remastered in a lot of cases. Things that made certain classes unique were obliterated and they started adding tedious things instead (like ridiculous numbers).
Gear dependency is also exaggerated on Cata. Basically how skilled a player is matters less in this expansion.
Cata added a lot of nice features, but they destroyed a lot of what made WoTLK appealing.