We decided that the community was too split to offer just one method of payment, so we decided to offer ALL three major methods. Here is the short version of the details of this:
B2P (like Guild Wars)
P2P (like WoW)
F2P (like LOTRO)
A player may choose to pay any way they want, and even change between them at times.
B2P pays no monthly, but must pay for expansions.
P2P pays monthly, but does not pay for expansions.
F2P has no monthly, but must either earn or buy credits to get certain items and areas. Earning them is possible, but very slow, most players will buy some credits and spend them on what they want.
B2P and P2P players may go to any server at will (even F2P servers), but F2P players may only go on F2P servers. While on a F2P server, B2P and P2P players will still have access to everything without need of credits, but this way they can play with their F2P friends.