In WoD we had alot of system overhauls, new character design and texture upgrades. Also testing of new types of content and technology.
I believe (hope) that WoD was a "test expansion" to break in previously mentioned.
With all those aestetic upgrades swallowing up development time It was no wonder they didnt have time to make that much content. Now, with all the new techs being used and reviewed, Blizzard now has everything they need to pump out alot of additional content as no development time is spend on new systems, char design, etc. anymore.
So do you think WoD was a testdrive and next expansion will build on the experience gained and pump out alot of content?
TLDR: Blizzard spend alot of time on creating new chars and shit. Now they dont have to anymore = more time to develop content?