As Revealed by WoW Insider, World of Warcraft: Warlords of Draenor will become available on or before the 20th of December this year.
They have found a line in the pre-order for the expansion content that explicitly says “Warlords of Dreanor is expected to release on or before 12/20/14.” The exact phraseology of “on or before” suggests that this will be the latest date that the expansion will be released, meaning that it could come out even earlier.
Indeed, Adam Holisky at WoW Insider thinks this may well be the case: “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).”