WotLK was just the continuation of what BC started:
-Streamlined questing with big quest hubs
-Small, digestable dungeons to farm
-Well balanced raids of smaller sizes where everyone needed to play optimally
-Even more emphasis on instanced PvP
-Progenitor of LFD system makes its appearance
-Every "spec" becoming balanced and optimal for raids
Anything WotLK did to go away from vanilla it was just a continuation of BC changes.