Looking for Group free on YouTube now. http://www.bit-tech.net/news/gaming/2014/11/10/wow-documentary/1
That documentary was ok. I felt they could have focused more on the errors made with each expansion, and how they learned and made changes. For example, Cataclysm is generally regarded as the weakest expansion; it would have been nice to hear more about how they took those (many) design errors on board and moved forward.
Wotlk was not the weakest, by far. it was one of the best actually, maybe tbc beats it for some. I thought the documentary was pretty predictable but it was good to see glimpses of whats happened behind the scenes, both recently and in the vanilla era.