There's a brief World of Warcraft Q&A on Polygon where lead designer Tom Chilton spoke with them at Gamescom about how embracing more casual players was a key element in the phenomenal success of World of Warcraft. They discuss if this will ever extend to making the game free-to-play, and he says they are not closed to the idea, though he expresses quite a bit of uncertainty about the prospect:

"For Blizzard it makes sense [to go free-to-play] at some point. But a lot of the risk is in making that transition. You her stories about developers going free-to-play and getting double the number of players, but you don't always know it works out that way and how long it stays that way. We really don't know what the rate is before people drop off and lose interest."