If I wanted to steal players from WoW and released a new MMO, I think what I would do is offer a trial from day 1. The trial would cost a one off fee of say, $5 - you would download the client and gain access to a couple of trial servers. That way the main servers populated by the player base aren't hindered. The trial accounts would be full and have access to all features up to a certain level, say 10. However they would last 7 days. After that point you can either upgrade for full price, or not. You only get one trial per credit card.
That way players thinking 'Hmm what's this new game' can throw $5 at it and see what it's like. Take a punt on it. If they don't like it, they're only $5 worse off. If they like it, they upgrade.
However, I suspect the thinking is that the 'hmm I want to try that' will be so strong that people go out and buy the full version just to try it out.. and the profits are bigger from that carrot on a stick. But they could still bring in some income from $5 trials too.
Just my 2c worth. No real point to this thread other than thinking out loud.