I was reading through twitter (like you do) when I came across an interesting conversation involving Jeremy "Muffinus" Feasel - he asked the question how long is too long for an expansion intro? Well, he asked it better than that.
Cataclysm and Mists of Pandaria continued this to some extent - if you were a level 80 character, you didn't get much more introduction than 'bad stuff is happening, you can go here or here' but the two zone options, Hyjal and Vashj'ir, were very detailed and had extended sections of on-rails questing. Cataclysm also had two complete starting zones for worgen and goblins that served as introductions to the meat of the expansion. Mists had the pandaren starting zone, but it also had the Jade Forest which had an outright introductory feel that was a lot stronger than any zone introduction had ever been, combining elements of the DK starting zone and the Vashj'ir start.