Simply the best MMORPG to date.
Most people who play World of Warcraft complain that Guild Wars has a low level cap at 20. This is one of the reasons that make is such an amazing game. World of Warcraft focuses its gameplay entirely on constant grinding and leveling. Playing a game like that gets boring extremely fast. Guild Wars focuses its gameplay on completing quests, finding items, armor, and other materials. Most people who want to play a quality MMORPG don't play for grinding, they play for the quests, which is what Guild Wars offers.
The most important factor that makes Guild Wars a much better game than World of Warcraft is its administration. Arena-Net provides a well-rounded environment for its players, prohibiting practices such as bot-farming and the purchasing of in-game gold. Blizzard, however, supports these acts and takes no action to regulate them. Arena-Net is more devoted to providing a proper gaming environment than Blizzard can ever hope to be. With the administration comes in-game admins. In this respect, Arena-Net admins take care to read any report sent and take the proper action required to rectify the situation. Blizzard admins, sadly, abuse their power however they see fit. Weather it be by randomly killing a player for speaking out, or by permitting bots to run rampant, Blizzard is corrupt.
The bottom line: Guild Wars is the better game with the devoted administration behind it, and no other MMORPG can come close to matching its standards.