Lots of factors go into it. Most prominent are Quality and Quantity. The Quality of the product mostly, better things require longer (also better and more skilled coders produce better work). and Quantity being how much... how many people are working on it, how many lands or areas, how many features or items (to some extent that also falls under quality).
they have been working on it for a while. 2 years? I think its something like that. But posibly it was just an idea kicking around for quite a bit of that. But how many people were helping on it? I mean, gaia does have to mantain the site and do the art for everything else that comes out, so we cant expect that there were too many people working on it. Maybe a few dozen. World of Warcraft was in development for 3 years before it was released wasnt it? or maybe it was more. Granted Blizard already had some experience with games and getting pieces to move around on the field, so some of it was second nature.
I really hope its better than the Gaia towns anyways, the controll and colision detection are just terible. Almost like playing the first warcraft game.