With the successful transition of Dungeons and Dragons Online to the free-to-play model, there's a lot of talk about why other games don't move over to the same model, with Warhammer Online frequently being cited as a game that would enjoy a great benefit from the switch. It would bring in more players, certainly, but there's more to the business than just bringing in new blood. The Ancient Gaming Noob makes an excellent point in a recent entry regarding the inescapable paradox of any game offering a free-to-play model -- that razor-edge split between giving the game away for free and making it entirely unreasonable to play it for free.
The obvious idea is that more people will try the game if they don't think they have to commit, and if they feel they can keep playing without having to pay money they're more likely to stick with it. However, every player that isn't paying money is essentially a cost - and as the entry points out, your paying playerbase will frequently be a small percentage of your overall subscribers. The entry uses Battlefield Heroes as an example of a game where the equillibrium needed to shift to make it profitable, which is causing serious rumbling in the gaming community, but the example can apply for any free-to-play game. As the model becomes more and more popular, it's important to remember that it's not a certain hit -- and can turn what could have been a moderately-successful subscription game into a failed game if done wrong.