Again it wasn't, just the start, TBC was the best expansion numbers wise, Wrath maintained it well though.
It's not comparable in any way shape or form though, vanilla and TBC was linear expansions, Wrath and beyond has been segmented, it's two very different designs where my personal preference is the linear that allows for hard to reach goals far off in the distance motivating your game play, segmented leaves you with the feeling of "why bother" especially when coupled with gradual nerfs in an effort to transform difficulty settings into "content"
I don't disagree, there are some major design issues where Blizzard has given at least half the arm though and now they can't take it back. What killed the game most for me was the changes to raids making 10 and 25 share lockout and making them "equal" as it removed lots of content and the nerfs mentioned above that only really benefit players craving insta gratification or free loots.
That and LFD/LFR hate both with every fiber of my being and if I could go back and uninvent them I would in a heartbeat, sadly it's to late now as it's to convenient to take back.