Honestly I think they killed themselves with cataclysm.
Honestly, they killed themselves slowly with WotLK and Cataclysm was the final blow.
They both made the game go down instead of up. They have some good things, but frankly they never actually sat down and bothered to work on so many things in the game that needs to be fixed and enhanced. Its still the same broken, limited MMO and has lost most of its appeal by now.