A line in the pre-orders to Warlords of Draenor tells us the expansion will be released on or before December 20, 2014. This is the second real hint of when Warlords will release, and it's inline with the previous information we've had, that it will be released in 2014. However, the key here is in the language:
on or before
I decided to bold that so there is no confusion. "On or before" means that the game will not be released any later than that. But my bet? It's going to come out a lot sooner. A release in December would mean a full year without any new content, and Blizzard has said multiple times it has no intention of doing that. They have said multiple times they're aiming for a release Fall 2014 (which technically, December 20 is the last day of Fall).
We've reached out to our contact for additional comment, and will update this post accordingly if we hear back.