free-to-play business models can prompt a lot of venom from players who are certain that the model, essentially, is a trick. You'll be forced into paying for the game somehow, or you'll be able to pay extra money and walk away with all of the best stuff in the game. Raph Koster has a different view of things from a business standpoint, and he outlines how the free-to-play model works out in practice in a recent blog entry.
Koster explains that you wind up with a handful of players who sink a huge amount of money into the game, while most users (upwards of 60%) never pay a dime. The users who do pay that much money, referred to as "whales," are essentially being treated as if they're in a retail store, compared to the more traditional model where you pay $60 for a game only to find out you don't like it. Whatever your views on the model, it's an interesting look at the changing face of monetizing games and is well worth a read.