In my opinion if your going to go with Free-to-play, it's usually pretty smart to release frequent content packs that players have to pay for. I know, I know, It kind of defeats the whole Free-to-play idea but it does give the developer incentive to release more quality content than say a game like Runescape. I can see this game easily going the way of Runescape(but with better graphics and mechanics obviously) unless they're really careful with it.
One major MMO that has followed that business model and it has worked beautifully for them: Guild Wars.
Guild wars is still my favorite MMO and they run a free to play service. Just, when they release new content they do it in the form of an Expansion pack, that you can purchase from their online site or in stores. I think this still helps with the "Free-to-play-alot of 10 year olds" problem. I've played Guild wars for about 3 years and can only think of about 3 instances where i was seriously annoyed at someone being immature.
Time will tell i suppose.