GuildWars is great, but it's hardly a real MMORPG. The only times when you actually see a "massive" amount of people at the same time is when you're in towns. When you exit them, you get an instanced dungeon, that is, a map exclusively made for you and your party. But, if you don't mind that (that is, the "closed" world), it's a great game. Many items, classes and skills. Besides, each of the campaigns can be played as a stand-alone chapter (except for Eye of the North, which needs Prophecies [the original game] to be installed) and provide solid storylines, something which is quite uncommon in this kind of games.
If you're looking for something more generic, try 2Moons and ArchLord, both of which are free to play, the former being only available to players from the US.