Developer Isaiah "Izzy" Cartwright addressed this process in Guild Wars 2 development today in the newest ArenaNet blog post about attributes and iteration. Balancing these two aspects of combat, even with Guild Wars 1 combat to build upon, is a long process of trial and error as well as planning.
For example, a popular weapon choice for a Warrior character was a bow and sword combination -- a weapon set that the player could switch back and forth between as necessitated by combat. Unfortunately, the attribute system as originally created made this a hindrance, forcing the player to divide his attributes between strength and agility, creating a mediocre setup at best. This is one of several examples Izzy gave in the blog post.
ArenaNet will reveal more information at PAX later this week, but this serves as a detailed preview for those eager to know more. You can read the full story at the ArenaNet blog.