Despite the negativity this expansion gets, this was where dungeons took a step in the right direction. Mechanics were more prominent, Crowd Control was actually required, each class was more well-rounded for encounters.

As for everything else this expansion changed... Welp lets not pretend like it was all good before this. Each expansion (+Vanilla) before it tore at what WoW was and the different reasons people liked it.

Before Cata, Vanilla had lackluster lore around raid bosses.
Before Cata, TBC changed raiding for better or worse. (Depends who you ask.)
Before Cata, Wrath really turned questing into what felt like a chore instead of a necessary process.