Interesting...well, the basic premise is that the game is free for everyone to play, but there's some sort of limitation on it. Generally, there are 2 directions used: some levels/weapons/units/items are only available if you pay, but the more common one is that you can earn all of the available items...paying for them just saves you the effort. APB: Reloaded, for example, allows you to earn money in game to buy things or use real money instead.
If we're going to do it, though, we would definitely lean toward all items being earnable without paying, but there's a key difference from most games out there. As I stated in my Dev Blog, we wouldn't attempt to focus free-to-play around "maximizing profit", which is the driving factor that a number of games currently do with the earnable model. They do this by making it *very* difficult or time wasting to earn the best items, essentially forcing people to buy them. We would hope to strike the balance to allow most players to earn the buyable items by just playing the game.
There are some obvious tradeoffs here. On one hand, more people will be able to experience the game, because it's free. On the other, the game could become easier for those willing to pay more money, though we'd obviously do everything we could to keep this from happening.