The main limit of creating a collection of bookmarks and tagging them lies in your interpretation of bookmarks. A bookmark from a formal point of view is an URL, but actually for the user in function of the context it can be a to-do, an idea, a goal...

You can have tags for types, along with "usual" tags, but the problem is behavior. You need links semantically classified as different to have different behavior. That is what Licorize does, more than Delicious and usual bookmarking services do.