I actually watched a small gathering video from GW2 tonight, because my earlier response was based on my opinion as an experienced mmo player. What I saw in the video was a lot of nodes. And they are instanced. I dunno how many materials you need for something basic, but walking around an area will get you a lot of material for sure. I don't know what the respawn times of the nodes are, they didn't say. So, after watching the video I can safely say that the concept is cool and could work, if properly balanced.