The way leveling XP gain works in crafting is this: For leveling a discipline from 0-400, you will gain 10 levels along the way. By maxing out all 8 disciplines, you will gain 80 levels. That means you could dedicate a character to crafting, feed it all the mats you get on other characters and level it all the way to 80 without ever needing to kill a thing. As hardcore crafters, we think that is pretty cool.
— Linsey Murdock (ArenaNet Game Designer), Guild Wars 2 beta forums
To note, Guild Wars 2 lets you use 2 crafting skills at a time, but you do not lose progress in a skill when you switch to a new one. There is a cost to switch which two are active, scaling to your level in the crafting skill. : Zubon