I really like the idea. What do you think about a bookmarking site similar to google+? or maybe an add on? Wouldn't it be a neat feature to just "pull" the urls into "bookmarking-circles"?
Hmm. What you're saying sounds cool, but I don't think I actually fully understand what you mean. Are you talking about more sharing between collections, and sort of adding other people's links to your own lists? I think if people start to really use the service, a lot of cool stuff would become possible. I'm definitely interested to hear more if you have ideas!