Guild Wars is on my short list of MMOs that I wouldn't change anything about. Or, well, hardly anything. The character creation and customization is pretty robust, the combat is quick and lively, and the environments are superb. There's probably a pretty good storyline, too, but I always skip the cutscenes and neglet to read the quest text. Hey, I'm trying to play a game, not read a book. When I feel like reading books, I'll crack open a book thank you very much, give me some more gigantic animated plants to slay.
One of the very best things, in my opinion, is the art of Guild Wars. It has a very specific style, and a very epic feel. Recently, the developerts of Guild Wars (ArenaNet) released some concept art of environments that, to my knowledge, never got used. It's safe to say they wouldn't appreciate it being used in a commercial product, but there's no reason you still can't admire a little art for the sake of art.