Added new multiplayer features, and a ton of new quests, plus new professions.
My biggest gripe on it would be the questing structure in and around Kaineng City. It has the player running back and forth on maps repeatedly over exactly the same territory, making for what feels like a bunch of tedious chores instead of fun quests.
Nonetheless, it makes up for it later, with epic battles against dragons, giant sea monsters, and weird snake creatures called Naga that drop...of course...Naga Hide. The game does have a great sense of humor, with some humorous quests stolen from well-known stories.
One thing I find interesting about how GuildWars has added new races with their expansions. The first game had mostly white people as characters, and Factions characters have a very Asian appearance. The architecture in Cantha is very Asian, and the backgrounds are quite immersive.
Of course, if you get into GuildWars, you'll want them all. It's just that kind of game.