Valve made 12 times higher revenue by going Free2Play with TF2.

Many "smart" things could be said here, but I think this speaks for itself. I only want to leave the following two quotes here:
Building large updates with additional content was key to maintaining player interest and expanding the player base.
In order to make a successful free-to-play game, Valve needed to make sure that its players were constantly coming back to Team Fortress 2 and checking back in. Part of its strategy entailed engaging its player community in as many different ways as possible ....... "Pretty much every place you give the community a chance to change the game, they'll do it, and they'll probably do a better job than you would"
edit for the people who get this all wrong: please note that I actually like QL, and I think it's a better game than TF2 and this is not about hats