^Too small; DR
Yeah, the Cata expansion ruined the old zones in one detail: Too linear. They are basically now one long quest chain. If you want to enter a zone just to pick up that single quest line with the nice gear reward your SOL.
To answer the previous question. At it's height, WOW had over 12million players. 10 TIMES more than most other MMO's combined. I think WOW is now sitting at around 10 million players.