I think that the amount people decide to pay in a F2P game is highly dependent on what their friends are paying.
If you have friends in the game and they are mostly playing for free, you’ll feel like an idiot if you pay. If your friends are mostly buying a few things, you’ll be encouraged to do the same. If you don’t know anyone else who plays, or haven’t made any in game friends then chances are that you will only be playing until the next game catches your eye anyway. (Unless it offers a stellar single player experience which is not usually the case.)
So one goal for a F2P developer might be to nudge new players to engage socially with the more hardcore who are already paying.
The LOTRO F2P strategy of having both F2P and subscriber players on the same server might prove to be very smart indeed.