Guildwars isn't a full-on MMORPG. The devs have classified it as a CMORPG(competitive multiplayer online RPG) or somesuch, I forgot the exact term they used. The game is actually being made by Arenanet, a bunch of ex-Blizzard employees who were the creators of Battlenet and the Diablo games. NCsoft is just the publisher, so the games aren't using the same engines. L2 is using the Unreal engine of all things, and Guildwars and CoH are using their own custom built engines.
Anyway, GW won't have any monthley fees because Arenanet as managed to stream-line the network code enough that the money brought in from expansion packs will be able to cover the maintence costs.