I like the concept! Tags have never made sense to me because I may describe very similar things using very different tags depending on my mood that day, what I happen to be thinking about, or any other external influences. But if there are designated "tags" for each concept then your bookmarks really are better organized.
yes, me too, I have been using del.icio.us for 1+ year and now I have a gigantic un-useful tag cloud. Your app will not solve the problem of having too much tags, but at least make tags more meaningful.
I would suggest you to integrate it with del.icio.us in some way because I don't really feel like switching to another bookmarks system at this point.
Definitely a good job overall!