Blizzard's senior VP of game design Rob Pardo revealed at the Paris Game Developers Conference that World of Warcraft was originally intended to be a free-to-play, advertising supported MMO. Pardo said, reacting to the impact of free MMOs in Asia on WoW's success, that "market conditions" forced them to realize that a subscription based model was the way to go.
While we're certain that Blizzard is enjoying the 10 million or so World of Warcraft subscribers forking over $13 to $15 each month in fees, it would be interesting to peek into the alternate universe where WoW was funded by ads.
While we're on subscriptions, Pardo said that one of the reasons behind MMOs failing to take off on consoles is the bite that console makers want to take out. In addition to a "lack of hard drive space, and difficulty in certifying patches" taking a percentage off the top is, literally, laughable to Pardo. Many more Pardo-isms at the full report.
Paris GDC: The Rob Pardo Experience [Gamasutra]